LTE - Observatoire de Paris - PSL - Centre de recherche en astronomie et astrophysique

Jean-Luc Margot's welcome at the LTE

Sylvie Lemaître — 2026-07-03T13:26:33Z

In September 2026, the Temps Espace Laboratory will welcome Professor Jean-Luc Margot, holder of the Tocqueville-Fulbright Chair of Excellence at the Paris Observatory-PSL. The aim of this chair is to establish, develop and strengthen Franco-American cooperation.

Jean-Luc Margot, a professor at the University of California, Los Angeles (UCLA), is an astronomer of international renown for his work in planetary radar astronomy, high-precision measurements of the rotations of planetary bodies, and the search for technosignatures. He is set to become a member of the Royal Academy of Sciences, Letters and Fine Arts of Belgium before the end of the year. He also heads the UCLA SETI initiative and has worked with over 200 students and 40,000 volunteers in the search for life in the universe.

From September, he will be working within the Astronomy and Dynamical Systems (ASD) team, alongside N. Rambaux (Sorbonne University) and J. Laskar (CNRS), on the study of Venus – the target of the forthcoming Envision mission – as well as on the study of icy moons and small bodies in the Solar System.

He will be giving public talks on the search for technosignatures and on high-precision radar measurements of planetary rotation, which you will be able to find in the newsletter and on our LinkedIn page shortly.

Jean-Luc Margot will finally be giving a lecture as part of the series of lectures at PSL Week on 23 November.

Learn more

Related projects by the ASD team : Envision (ESA), ANR ROCHE
https://psl-week.psl.eu/
https://seti.ucla.edu/jlm/
https://fulbright-france.org/fr/bourses/institutions/chaire-tocqueville-fulbright

A system comprising at least six exoplanets has been discovered following ten years of observations

Sylvie Lemaître — 2026-07-03T12:31:55Z

An international team led by Salomé Grouffal (Aix-Marseille University and Grenoble Alpes University), including members of the LTE, has spent ten years studying an exceptional planetary system comprising at least six planets with long orbital periods. The orbital period of the outermost planet (around one and a half years) is of the same order of orbital period of Mars. Such a rare system offers a new insight into the diversity of planetary systems.

The planets in the HIP 41378 system pass in front of their star (the transit method), resulting in an almost perfect alignment with Earth. However, only one in every 200 systems like HIP 41378 has this configuration. The HIP 41378 system is therefore a rare gem, enabling the study of planets with orbits comparable to those of the planets in the Solar System.

Five planets in the system were first discovered in 2015 by the Kepler telescope, but little was known about their nature and orbits. Understanding this system required a decade of observations using instruments mainly based in Chile, which measured variations in the star's velocity caused by the planets (the radial velocity method).

HIP 41378 is thus one of the first multiple-planet systems in which long-period planets have been well characterised. It consists of three inner planets smaller than Neptune and three outer planets, which are low in density and may be as large as Saturn. A seventh, more distant planet has yet to be confirmed.

Among the exoplanets, HIP 41378 f is particularly intriguing: its extremely large radius, given its mass, defies planetary formation models. One hypothesis, currently being investigated at the LTE as part of the WRAPS project (“Where do Rings Appear in Planetary Systems?”), suggests that this planet is in fact surrounded by highly inclined, opaque rings, giving it the appearance of a larger planet (see Figure). The formation of such a ring could be the natural result of a moon's evolution around this planet.

Figure: Artist's impression of the system around the star HIP 41378. The rings around HIP 41378 f have not been confirmed. Credit : Salomé Grouffal

Learn more

Read the CNRS press release.

Read the original article : A decade of monitoring the HIP 41378 planetary system: Masses and orbital periods of six planets and a planet candidate, Astronomy & Astrophysics n°710, A275, 2026.

New release : Trois éclipses en lumière

Sylvie Lemaître — 2026-06-19T12:48:56Z

The Space Service (SE-OP) of LTE is publishing this illustrated book to observe and understand everything about next solar eclipses partially visible in France.

And to ensure you can view the eclipse safely, special eclipse glasses are included.

View The table of contents.

See some page extracts.

The book is available from the publisher via this link, or in bookshops.

Format 14 × 21 cm – 88 pages
Editor : EDP Sciences
ISBN : 978-2-7598-4018-2
Publication date : 9 juillet 2026
Prix : 12,90€

ÉclipSEOP: a web app to observe the summer eclipse

Sylvie Lemaître — 2026-04-17T13:34:06Z

To mark the astronomical event of 2026, the Service espace of Paris Observatory (SE-OP) is expanding its range of services for the general public and launching a web application, ÉclipSEOP, dedicated to observing the solar eclipse on 12 August 2026.

The ÉclipSEOP app provides access to all the information you need to observe the eclipse from anywhere on Earth. It is very easy to use, and its intuitive interface allows you to view the timings of the eclipse's various phases and a graphical representation of the phenomenon as it will appear at a specific location, all with just a few clicks.

Once you have selected a location on the interactive map showing the eclipse's path of totality, you will be able to see the type of eclipse (partial or total) visible from that location, the time of maximum eclipse along with the magnitude and degree of obscuration, its duration, and the duration of the totality phase if it is visible.

The local conditions of the eclipse and the times of sunrise and sunset at the chosen location are presented in chronological order, making it easy to understand the visibility of the eclipse's various phases. The Sun's altitude and azimuth at key moments during the eclipse are also provided.

Times shown in the app are in Universal Coordinated Time (UTC). To find out the time in mainland France, add 2 hours.

All data is provided by the Service Espace of the Paris Observatory (SE-OP), which compiles and publishes ephemerides for the bodies of the Solar System.

Learn more
• Link to ÉclipSEOP app
• Link to the SE-OP calculation forms portal

Numerical simulations of rings around irregular bodies: the unexpected role of resonance 1/3

Sylvie Lemaître — 2026-04-10T13:41:01Z

An international team, comprising a researcher from the Paris Observatory – PSL, Sorbonne University and the CNRS, and a researcher from the University of Oulu in Finland, has studied ring dynamics using N-body collision simulations. These simulations show that the 1/3 spin-orbit resonance can effectively confine rings around irregular bodies such as Chariklo by transferring the energy provided by this resonance to free Lindblad modes, a process never previously observed. This work is the subject of a paper published in the journal Astronomy & Astrophysics on 26 March 2026.

In recent years, rings have been discovered around small bodies in the solar system, such as Centaur-type objects like Chariklo and Chiron, or trans-Neptunian objects like Haumea and Quaoar. Unlike the giant planets, which are practically axisymmetric, these small bodies are irregular, creating strong spin-orbit resonances between the central object and its rings. In fact, several of the rings discovered are close to a 1/3 spin-orbit resonance with the body, in which the body completes three rotations whilst a particle in the rings completes one orbital revolution.

This is not the first time that resonances have been linked to the presence of confined rings. In fact, several of the rings of Saturn, Uranus and Neptune are confined by first-order m/m-1 resonances with a satellite, whereby the satellite completes m-1 revolutions whilst the particle completes m. These resonances, known as Lindblad resonances, were studied in the 1960s in the context of galactic dynamics. They force the formation of spiral arms in the disc, and in the case of rings, they can lead to the confinement of narrow rings.

In principle, the effect of a second-order m/m+2 resonance (i.e. 1/3 for m=1) is destructive. Indeed, unlike Lindblad resonances, the 1/3 resonance causes the current lines forced by this resonance to cross, a highly dangerous situation for a collisional ring. It is as if motorways were being built that cross without traffic lights! This is illustrated in the figure opposite, where we see the central body with a mass anomaly (small circle attached to the central body), surrounded on the left by the periodic orbit of a particle in a 1/2 spin-orbit resonance with the central body. This orbit does not intersect itself, unlike the periodic orbit corresponding to the 1/3 resonance on the right, which has a self-intersection point (blue dot).

The authors of the paper used N-body simulations, in which tens of thousands of particles with radii of a few tens of metres undergo inelastic collisions, whilst being perturbed by an irregular central body. This irregularity is modelled by a mass anomaly representing, for example, a mountain or a crater on the surface of Chariklo; see the figure above. This mass anomaly is quantified by a dimensionless parameter µ, which measures its mass relative to that of the central body.

Following an excitation phase that leads to the self-crossing of the current line excited by the 1/3 resonance, a surprising phenomenon occurs: the ring transfers the energy it receives to free Lindblad modes, thereby eliminating the self-crossing of the orbits. Even more surprisingly, these free modes ultimately lead to the radial confinement of the ring.

This process is illustrated in the figure above. It is the result of an N-body simulation involving 40,000 particles with a radius of 25 metres undergoing inelastic collisions and perturbed by the 1/3 resonance. The positions of the particles are shown in a longitude-radius diagram, with the dotted line indicating the radius of the 1/3 resonance. In the case of Chariklo, the radius range covered here (from 2.03 to 2.12) corresponds to a radial distance of approximately 18 km centred on the orbital radius of the resonance towards a radius of 2.08, corresponding to 400 km for Chariklo. The time shown is in units of Chariklo's rotation period (approximately 7 hours), i.e. approximately 130, 136 and 205 years for the left, centre and right panels, respectively. On the left, we see the current line forced by the 1/3 resonance, with self-crossing, followed by a transition period in the centre panel, which leads on the right to a strongly confined ring, which now avoids any self-crossing. Fourier analysis of this ring shows that it results from the superposition of Lindblad free modes.

By varying the mass anomaly µ and the particle radius, and using dimensional analysis, the authors show that Chariklo's main ring can be confined with mass anomalies of the order of µ≈10⁻³, assuming particles of the order of one metre in size. This value corresponds to a mountain (or crater) approximately 10 km high (or deep) on the surface of Chariklo, which is physically plausible for a small body with a diameter of approximately 250 km.

Votre navigateur ne supporte pas la vidéo HTML5.

This animation shows the confinement of a ring around the 1/3 resonance with Chariklo. It is the result of an N-body simulation involving 40,000 particles with a radius of 25 metres undergoing inelastic collisions and perturbed by the 1/3 resonance with Chariklo, with Chariklo completing three rotations whilst the particles complete one orbital revolution. The ring's arc is shown in perspective, with the distance on the x-axis (2 to 2.12) corresponding to approximately 24 km. The elapsed time in years since the start of the simulation is shown at the bottom right of the image.

This mechanism could also apply to the rings of Haumea and Quaoar, given that the latter also has a ring that is close to a 5/7 resonance with the central body. Like the 1/3 resonance, it is of order two, and therefore has the same topological structure, with self-crossings of the periodic orbits, and possibly the same behaviour as Chariklo's ring.

Reference

The article is published under the title : "Rings around irregular bodies II. Numerical simulations of the 1/3 spin-orbit resonance confinement and applications to Chariklo" dated 26 March 2026, https://doi.org/10.1051/0004-6361/202556946.
Another article setting out the theoretical aspects of resonances, by Sicardy, Salo, El Moutamid, Renner and Souami, was published on 25 November 2025, https://doi.org/10.1051/0004-6361/202556950.
This research was partly funded by the "Roche" project of the French National Research Agency ANR-23-CE49-0012. It is the result of scientific research carried out in France, at the Paris Observatory – PSL in Laboratoire Temps et Espace (Paris Observatory – PSL / CNRS / Sorbonne University / University of Lille) and at the University of Oulu in Finland.

Contacts

Bruno Sicardy, Professor Emeritus SU, LTE
+33 (0) 1 40 51 23 34
bruno.sicardy@observatoiredeparis.psl.eu

Heikki Salo, Space Physics & Astronomy Research unit, University of Oulu
90014 Oulu, Finland
Heikki.salo@oulu.fi

Poisson, Arago, Young, Fresnel, Sommerfeld, Pluto and Triton: a strange story of the central flash

Sylvie Lemaître — 2026-03-25T13:23:39Z

A French team, comprising a researcher from the Paris Observatory – PSL, Sorbonne University and the CNRS, and a researcher from Jean Monnet University in Saint-Étienne, has studied the structure of the central flashes produced during stellar occultations by the atmospheres of Pluto and Triton, which act as immense lenses focusing starlight towards Earth. This work is the subject of a paper published in the journal Astronomy & Astrophysics on 17 March 2026.

In the early 19th century, a number of experiments, including those conducted by Thomas Young and Augustin-Jean Fresnel, led to the triumph of the wave theory of light. However, not all scientists were convinced. Siméon Denis Poisson, for instance, a proponent of Isaac Newton's corpuscular theory of light, was staunchly opposed to it. The controversy intensified with a question put forward by the Academy of Sciences in 1818. On that occasion, Poisson had noted that one of the consequences of Fresnel's wave theory would be the presence of a bright spot at the centre of the shadow cast by a circular opaque disc, due to constructive interference of the wave. This struck him as absurd: how could light illuminate the centre of a completely black shadow? Especially since no one had ever observed such a spot… However, François Arago was able to produce one using a small metal disc. The jury members were convinced, and the Academy's prize was awarded to Fresnel in 1819.

In memory of these debates, the observed spot is now known as the Poisson spot, or the Poisson-Arago spot, or the Fresnel spot. It made an unexpected reappearance during calculations carried out to describe the ‘central flashes' observed during stellar occultations by objects such as Pluto or Triton (see figure opposite). The question was: what is the structure of the flash produced by a perfectly spherical and transparent atmosphere? Can its amplitude be infinite? What role does diffraction play?

The calculations describing the flash involve path integrals and make use of Sommerfeld's lemma, which is fundamental to quantum mechanics. This is only logical: photons are, by their very nature, quantum objects! Building on this, the authors consider several scenarios, starting with a spherical occluder without an atmosphere, of radius R₀, and a star assumed to be point-like. This yields the classical Poisson spot, whose maximum irradiance is equal to that received from the star outside the shadow. The Poisson spot is described by the square of the Bessel function of the first kind and zero order, J₀. It is an oscillating function that reaches unity at the centre of the shadow and has a width close to λ∆/(2R₀), where λ is the observation wavelength and ∆ is the geocentric distance of the occulter. This peak is also surrounded by fringes, equally spaced at approximately λ∆/(2R₀).

The figure opposite shows the structure of the shadow cast by an opaque spherical body with a radius of 10 km situated at Pluto's level and observed in the visible spectrum. Note the classic Fresnel fringes on the left and right edges, caused by the object's abrupt and opaque limb.

If we introduce a thin atmosphere, but one too weak to focus the light rays towards the centre of the shadow, not only does the Poisson spot remain, but it is amplified by a factor of (R₀/r₀)², where r₀ < R₀ is the radius of the shadow cast by the body, taking into account the refraction of light rays due to the atmosphere of the occulter. The figure below shows the theoretical structure of the shadow of Pluto, whose atmospheric pressure has been arbitrarily reduced to one-tenth of its current value, and observed in millimetre waves. The Fresnel fringes mentioned above can be seen at the edges of the shadow, as well as the central Poisson spot amplified by a factor of (R₀/r₀)².

If the atmosphere is dense enough to focus the light rays towards the centre of the shadow, r₀ no longer exists. Calculations then show that diffraction imposes a finite flash height of approximately (2π)²RH/(λ∆), where R is the radius of the layer causing the central flash (close to R₀), and H is the height scale of the atmosphere. For Pluto and Triton (with 0.01 mbar at the surface), this flash height can reach very large values in the visible spectrum, ranging from 10⁴ à 10⁵ times the star's luminance outside the occultation. At the same time, the width of the flash projected onto Earth λ∆/(2R) is extremely small, on the order of a metre, making it impossible to resolve with current technology. However, as this width is proportional to λ, observations made at millimetre wavelengths could resolve the flash and the fringes surrounding it, which would then have sizes on the order of a kilometre.

The figure opposite shows a theoretical curve of a Pluto occultation observed in the visible spectrum. The Fresnel fringes are now too faint to be observed. Around the flash, which reaches very high off-scale values (10⁴), rapid fluctuations caused by Poisson fringes can be seen. These fringes result from interference between the primary and secondary stellar images produced by Pluto's atmosphere, and are therefore identical to the interference produced in Young's double-slit experiment, but with a subtle additional phase quadrature correlated with the fact that the light rays corresponding to the secondary image pass through the axial caustic just before reaching the observer's front plane.

That said, other effects must be taken into account when describing the flash. For example, the size of the occulted star is finite. When projected onto the occulting body, this size is usually of the order of one kilometre. This implies that diffraction effects are smoothed out by the stellar diameter: in the case of a thin atmosphere, the height of the central flash is then less than (R₀/r₀)²; in the case of a dense atmosphere such as Pluto's at present, calculations show that despite this smoothing, the flash can still reach heights more than 50 times the star's initial signal. Another effect to take into account is the possible distortion of the atmosphere relative to the spherical model. In this case, the Poisson spot is replaced by a caustic lined by fringes, a subject for future study…

Reference

The article is published under the title : “Central flashes during stellar occultations.
Effects of diffraction, interferences, and stellar diameter” on 17 March 2026, https://doi.org/10.1051/0004-6361/202555548

This research is the result of scientific work carried out in France at the Paris Observatory – PSL, in the Laboratoire Temps et Espace (Paris Observatory – PSL / CNRS / Sorbonne University / University of Lille), and at Jean Monnet University in Saint-Étienne, CNRS, in the Institut d'Optique Graduate School, and the Laboratoire Hubert Curien , UMR 5516.

Contacts

Bruno Sicardy
Professor Emeritus SU, LTE
+33 (0) 1 40 51 23 34
bruno.sicardy@observatoiredeparis.psl.eu

Luc Dettwiller
Contributor to Laboratoire Hubert Curien,
Institut d'Optique, Saint-Etienne
dettwiller.luc@gmail.com

Two summers and one winter, three rare events: the ‘Iberian trilogy' of eclipses (2026–2028)

Sylvie Lemaître — 2026-03-03T14:32:49Z

Imagine: within three years, the Moon will repeatedly ‘target' Southern Europe. As a result, Spain becomes the best place in Europe to experience exceptional solar eclipses – a total eclipse at sunset (2026), a total eclipse in the morning (2027), and then a spectacular ‘ring of fire' at dusk (2028).

12 August 2026: Totale, the ‘summer night' at dusk

Visibility map of the total solar eclipse on 12 August 2026. Credits: LTE

Movement of the Moon's shadow over the Earth during the total solar eclipse on 12 August 2026. The black dot represents the locations where the eclipse will be total. Credits: LTE

In the middle of summer, the Sun is about to perform a real disappearing act. The band of totality crosses northern Spain (and also passes through Iceland): there, for about 1 to 2 minutes depending on the location (max 1 min 50 s in northern Spain, up to 2 min 18 s on the central line before reaching Europe), the day changes - the sky turns blue, the air cools, and the solar corona appears as a living halo around a black disc. And it all happens quickly: the shadow moves at about 3,429 km/h, like a dark wave breaking silently. In France, it will be a partial eclipse in the evening, perfect for observing on the way back from the beach or a picnic: the light becomes strange, as if someone had slowly turned down the world's dimmer switch. And for ‘easily accessible' Europe, it's a highly anticipated event: the first eclipse visible from mainland Europe since 1999, the return of a great spectacle close to home.

2 August 2027: Total, the great ‘long'

Visibility map of the total solar eclipse on 2 August 2027. Credits: LTE

Movement of the Moon's shadow over the Earth during the total solar eclipse on 2 August 2027. The black dot represents the locations where the eclipse will be total. Credits: LTE

A year later, another dramatic turn of events... but this time in the morning, when the sky is clear and the shadow advances across the continent at a speed of 2,398 km/h. The totality grazes Europe via southern Spain, towards Andalusia and the Strait of Gibraltar: there, the darkness can last several minutes (up to nearly 5 minutes in the best areas: 4 min 49 s in Ceuta, more than 4 min 30 s in Tarifa), long enough to feel the silence descend and see the horizon take on the colours of twilight across 360°. Further along its path (particularly towards Egypt), the eclipse reaches a maximum of around 6 min 23 s – the second longest total eclipse of the 21st century and the longest over easily accessible land, said to leave ‘time to look', as if time itself were looking: time to be surprised, time to understand, time to get goosebumps. In France, it will be seen as partial, like a crescent nibbling at the Sun, but the event itself will take place a few hours' drive away. And to exceed this duration, we will have to wait a very long time, nearly a century: the next longest total eclipse is announced for 3 June 2114, with a maximum of 6 min 32 s, before the very long one on 13 June 2132, approaching the maximum duration of a total eclipse when all the optimal celestial conditions are met, i.e. 7 min.

26 January 2028: Annular eclipse, the “ring of fire” at dusk

Visibility map of the annular solar eclipse on 26 January 2028. Credits: LTE

Movement of the Moon's shadow over the Earth during the annular solar eclipse of 26 January 2028. Credits LTE

The final act, in the middle of winter: not a night in broad daylight, but a celestial gem. On 26 January, the Moon moves directly in front of the Sun without covering it completely: it leaves a bright, thin, sharp, almost unreal ring – the famous ‘ring of fire'. The ring-shaped band once again passes over Portugal and Spain (with a passage towards Gibraltar and northern Morocco) and, as an added bonus, the phenomenon occurs in the late afternoon: a twilight spectacle, where the ring can last for several minutes, with a global maximum of around 10 minutes, which is very rare for an annular eclipse and therefore exceptional (the maximum ‘theoretical' duration of an annular eclipse is around 12 min 35 s). Here, the relative slowness of the shadow is part of the magic: nearly 1,709 km/h - and it is also one of the main reasons why the ring can linger for so long. In France, it will be a partial eclipse: a foretaste. But in Iberia, with a clear horizon, we will be able to see the Sun wearing its ring – as if the sky were signalling, in light, the end of this trilogy.

Learn more

Total solar eclipse on 12 August 2026
Total solar eclipse on 8 August 2027
Annular solar eclipse on 26 January 2028

The EQUIP-G project takes off

Sylvie Lemaître — 2026-03-03T13:48:42Z

Towards a permanent European entity managing a shared fleet of quantum gravimeters accessible to all and a network of ground reference stations.

Gravity measurements are a unique tool for subsurface imaging, as they provide direct information on underground mass variations. They also play an essential role in establishing terrestrial spatial references. Gravimetry can be used to map and monitor subsurface dynamics, and has numerous applications: geothermal energy, monitoring climate change, groundwater and volcanoes, and underground gas storage.

After decades of research, development and industrial transfer, quantum technology has reached a high level of maturity and it is now possible to benefit from operational quantum gravimeters in the field, offering numerous advantages over the devices used until now.

Quantum gravimeter deployed in Greenland in August 2025 as part of the EQUIP-G project.
Credits Tim Jensen, DTU

The EQUIP-G consortium is deploying a fleet of quantum gravimeters across Europe and offering free loans to the geophysical and geodetic community to use this technology. By 2029, the project will provide the European Commission with details and recommendations concerning a future fleet of shared instruments (terrestrial or onboard quantum gravimeters). This will involve managing requests from various public institutions, monitoring the instruments and providing training. Finally, it will also involve finding the best solution for storing and making available all the data, which will be public.

The future entity would be similar to a research infrastructure, with challenges similar to those faced by astronomers who reserve observation time and collect data when sharing telescope time.

How does a gravimeter work?

A quantum gravimeter is a sensor that replicates Newton's apple experiment. Several times per second, atoms are trapped and cooled by laser to form a small sample of matter. This test mass is then released in free fall into a vacuum. The acceleration experienced by the atoms is measured by a vertical reference laser using the principle of atomic interferometry, which allows precision measurements to be made by interfering waves of matter manipulated by laser pulses. The result corresponds to the acceleration of Earth's gravity, g.

The LTE laboratory's LNE-OP service was one of the pioneers of interferometry worldwide in the early 2000s, and part of this research was successfully transferred to industry in the 2010s. Active research on the subject continues to be conducted by the Atomic Interferometry and Inertial Sensors team.

Who is behind this project ?

Following a call for proposals from the European Commission (HORIZON-CL4-2024-DIGITAL-EMERGING-02), the EQUIP-G project, comprising a consortium of 20 partners from 11 European countries, has been selected to develop and deploy a network of quantum gravimeters in Europe by 2029. The project is led by the CNRS and headed by Sébastien Merlet and Jean Lautier-Gaud, members of the LTE.

Learn more

The project website.
Atomic Interferometry Team Web Page du LTE.
Video, broadcast by ARTE, describing how the quantum instrument works on Mount Etna.

Publication: Les dentelles noires de l'Etna

Sylvie Lemaître — 2026-03-03T12:55:10Z

On 20 February, a book entitled Les dentelles noires de l'Etna was published, written by Jean Lautier-Gaud, a member of the LTE staff. In it, he recounts an experiment carried out by an LTE research team.

In this novel, Jean Lautier-Gaud recounts the scientific and human adventure in which he participated to install a quantum gravimeter at ‘Dentelles noires', the observatory station located closest to the craters of Mount Etna. This sensor, based on the work of the LTE's Atomic Interferometry and Inertial Sensors team, is used to study and monitor volcanoes. The author discusses the project and the novel.

« My thesis with Arnaud Landragin at LTE focused on developing a compact quantum gravimeter for field measurements within the Atomic Interferometry and Inertial Sensors team. In 2014, we published a major result paving the way for the use of these instruments outside laboratories.

The technologies developed by the team were transferred by the CNRS to the start-up Muquans, which turned them into a commercial product, enabling one of these sensors to be installed on Mount Etna, an expedition in which I personally took part.

I ended up writing a book on the subject, in which I revisit the human and scientific adventure of climbing. Popular science on the subjects of metrology, quantum sensors and geophysics features prominently in the book. The novel was released on 20 February in bookshops, published by Transboréal.

Today, the deployment of quantum gravimeters in the field continues, notably as part of the EQUIP-G project, supported by the CNRS and led by the LTE. »

Format : 12 × 18 cm – 224 pages
Editor : Transboréal
ISBN : 978-2-36157-366-9

Learn more
Le projet EQUIP-G
Link to the publisher Transboréal
Link to the article about the scientific work behind this experiment

What time is it on the Moon?

Sylvie Lemaître — 2026-02-17T13:38:33Z

Exploration of the Moon and its surroundings has seen renewed interest over the past decade, as evidenced by the Artemis (NASA), LunaNET (NASA/ESA) and Moonlight (ESA) programmes. Their objective is to establish a permanent presence around and on the Moon, which requires the implementation of infrastructure dedicated to navigation, positioning and telecommunications in the vicinity of our satellite. To do this, it is necessary to agree in advance on the use of a coordinated time scale, playing the same role as Coordinated Universal Time (UTC) on Earth, and enabling the various lunar players to exchange information and compare their measurements.

The concept of coordinated time is provided by the theory of relativity and should not be confused with the more familiar concept of proper time. The latter has a local dimension and represents the time physically indicated by an ideal clock. In contrast, coordinated time is purely conventional, but has a global scope, meaning that it is defined and can be used anywhere and by any observer. Two distant observers can thus compare the primacy of their respective local dating by converting (via a mathematical procedure) their measurement of proper time into the same coordinated time scale.

In the lunar environment, three coordinated time scales may be of practical interest. The first, E1, is the most fundamental time scale; it is naturally given by the theory of general relativity: it is the lunocentric coordinated time. The duration of one second of E1 coincides with one second of a clock located at the centre of mass of the Moon; E1 is thus the analogue for the Moon of geocentric coordinated time. The second scale (E2) is obtained by artificially applying a multiplicative factor to the duration of one second of E1, so that the second of E2 coincides with the second beaten by a clock at rest on the lunar geoid; E2 is therefore the analogue for the Moon of Earth time. Finally, the third scale (E3) is also artificially constructed by applying a multiplier to the duration of a second in E1, ensuring, this time, that the duration of a second in E3 is as close as possible to a second in the coordinated universal time scale.

The E2 scale may be advantageous if several clocks placed on the surface of the Moon wish to exchange their measurements. This is because the proper time of each lunar clock will remain close to the E2 coordinated time used for comparison, which masks the mathematical procedure of transforming proper time into coordinated time. However, as the surface of the Moon is very flat, an atomic clock will not generally be located on the lunar geoid; it will therefore not tick at the same rate as E2 and the mathematical transformation procedure cannot generally be avoided. This procedure must be applied if the coordinated time scale is E1 or E3. In all three cases, it can still be circumvented by artificially changing the frequency of the lunar clocks. The frequency corrections to be applied are of the order of 10-11, 10-13 and 10-10 (relative) for the E1, E2 and E3 scales respectively.

At first glance, E2 therefore appears to be the most advantageous. However, E2 and E3 are scales of E1, which, in the context of general relativity theory, implies that masses must also be scaled. This scaling of physical parameters is problematic in that the same mass can then be assigned several numerical values! For example, adopting E2 or E3 would require using a mass for the Earth that would not have the same numerical value as that defined naturally by the barycentric coordinated time scale.

In conclusion, since the mathematical procedure that transforms proper times into coordinate times E1, E2, and E3 cannot be avoided for clocks located on the surface of the Moon, and because E1 does not involve scaling physical parameters unlike E2 and E3, we recommend adopting the lunocentric coordinated time (i.e., E1) as the coordinated time scale associated with the lunar reference system. In the near future, this approach will be easily transposable to other bodies in the solar system, notably Mars.

Contact
Adrien Bourgoin, Astronome adjoint, LTE, Observatoire de Paris
adrien.bourgoin@obspm.fr

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