100 years after, Astronomers carry out the Einstein's Eclipse experiment at La Silla Observatory in Chile

May 29, 1919 was a great day for modern astronomy, especially as far as Einstein's physics is concerned. For the first time in history, the theory of General Relativity was verified for real: an experiment led by astronomer A.Eddington and based on observing the 1919 total solar eclipse.
Z. Hoder, M. DruckmĂĽller (http://www.zam.fme.vutbr.cz/~druck/), P. Aniol, S. Habbal / Solar Wind Sherpas (http://project.ifa.hawaii.edu/solarwindsherpas/) / ESO
100 years after, during the July 2nd 2019 Solar Eclipse in Chile, a team of french astronomers will reproduce this famous experiment, measuring the deflection of starlight in the vicinity of the Solar disk.
The ESO's observatory of La Silla is where this practical work will be conducted, using the 250-mm aperture telescope TAROT.
In this article, we invite you not only to get back in time but also to introduce the 2019 version of this symbolic experiment in the History of sciences. In order to browse more easily in this page, here is the interactive outline:

1919: Einstein's Eclipse or when the theory fits perfectly with reality

When the Einstein's theory of General Relativity was published in 1915, a majority of scientists were reluctant regarding its accuracy. A doubt which was legitimate since it questions the universal law of gravitation, which had been established by Newton and applied by scientists since 1687.
According to general relativity, every massive body in the Universe affects space-time by creating a distortion. Then, gravity does not have to be considered as a force anymore but rather as a curvature of space-time. Since laws of light propagation are governed by space, if the latter is distorted, so the light is.
Based on this principle, Einstein enumerated few experiments which could reflect his ideas. One of them was to observe the deviation of starlight as it passes next to the Sun. However, the only way to notice this slight variation requires to block sunlight. Such a configuration is only possible during a rare and unique phenomenon we all know as Total Solar Eclipse.

In 1919, two British astronomers, Eddington and Dyson were the first to carry out an experiment of such. They compared positions of stars close to our Sun during the eclipse to the exact positions of those stars without the sun nearby.

Results helped to prove Einstein prediction with a respectable accuracy. The exact values were ranging from 1.6 arcsecs to 1.98 arcsecs. Undoubtedly, this real-life test created a huge enthusiasm towards the Genius's theory, especially because it was the first consistent experiment to be conducted and optionally to check its exactness.

In 1922, a second test was made and again, verified that Einstein was right. No other records of reproducing this experiment were found until 1973.

Find more details about the 1919 experiment in this longer article version.

To celebrate the 100th anniversary of Einstein's Eclipse, a handful of french Astronomers decided to follow the steps of Eddington and Dyson during the July 2 2019 Total Solar Eclipse in Chile.

2019 : The centenary of Eddington's experiment honored by TAROT in Chile

The TAROT Telescope (foreground, right) and the ESO-3.6-m Telescope (background, left)
in total solar eclipse sky (simulation)

★ A Total Solar Eclipse over ESO's La Silla Observatory

The July 2 2019 eclipse is extremely rare. Why? For the first time in History, a total solar eclipse will be observed directly from professional astronomical observatories in Chile. Such a situation is a real asset since astronomers could study the solar corona  more in depth using one the largest ground-based telescopes in the world.
The observatory of La Silla is part of the lucky ones which could witness such a rare phenomenon. Located in the outskirts of the Atacama desert, the first-ever built European Observatory will benefit from having a total eclipse at 4:39 p.m on July 2 with a duration of 1 minute and 51 seconds.

La Silla is the eclipse-chaser 's spot of dreams and is where a team of astronomers will repeat the famous 1919 experiment. Among them, you might recognize some familiar names:

Christian Buil is a talented amateur astronomer who won
an international recognition award by the International
Astronomical Union in august 2018. His website
  • Alain Klotz, Astronomer and professor at IRAP (Toulouse, France)
  • Christian Buil, a reference in the world of amateur astronomy, optical instrumentation engineer at CNES (Toulouse, France)
  • Jean-François Le Borgne, Emeritus Astronomer at IRAP

★ The principle of the 2019 experiment

Alain, Christian and Jean-François will use the TAROT telescope for measuring the bending of light around the Sun. TAROT stands for "Télescope à Action Rapide pour les Objets Transitoires" or "Quick-action telescope for transient objects". Despite its relatively modest 25-cm diameter, the 850-mm focal length enables TAROT to be a premium choice telescope for capturing faint objects within a few amount of time, thanks to its high light-gathering power.
TAROT's wide field-of-view added with its fast-moving motorized mount make it a genuine Guardian of the sky. As its name suggests it, this telescope aims at studying short-timescale and low-brightness phenomena such as Gamma ray-burst (GRB) or supernovae.
TAROT telescope and New Technology Telescope NTT (extreme left, background). Crédit : ESO.

One of the requirements set out by the team of astronomers is that TAROT's configuration should not be specifically modified for the July 2 solar eclipse. 

The main advantage of using a permanent telescope like TAROT is to be able to capture reference frames indicating the accurate position of stars before the eclipse to occur. This calibration step is obviously mandatory since it should enable measuring the value of starlight-deviation angle caused by Sun's gravity. Consequently, the team already recorded images of the same region of a sun-free sky few months preceding the eclipse.

As soon as the Moon will entirely cover the Sun, at 4:39 p.m, the three french astronomers will be charged to take the most images as possible in order to detect a sufficient number of star in the vicinity of the Sun's corona. One of the main goals of the mission is to measure the deflection of light closest to the solar disk as achievable. Since deviation of light-rays is inversely proportional to the angular distance to the Sun, the bending effect will thus be higher as we observe closer to the Sun.  However, the nearer from the Sun we loot at, the less stars are visible in the sky. The reason is the increasing brightness of the Solar corona as we get closer to the Sun.
This physics issue was identified during the 1919 experiment carried out by Eddington and Dyson. Their photographic plates had at that time, a poor dynamic range and were far less sensitive as our modern CCD sensors. With this 20th century equipment, they should select stars distant enough from the Sun so that they were not flooded by the stronger brightness of the corona. In this case, the light emanating from these stars barely distorted because of their higher angular separation in respect to the Sun's gravitational field.

The outcome expected by Alain Klotz and his team will be deviation values narrowly located in the theoretical prediction of Einstein's general relativity, namely 1.75 arcseconds. Such a deflection is estimated to be measured at an angular separation of a half of the Sun diameter. Put it differently, measuring such a bent of light is equal to spot a smartphone from a distance of 16 km!

The images taken by TAROT telescope will also serve to astronomers operating the 3.58-meter New Technology Telescope, who will simultaneously conduct spectroscopic observations of the corona. The role of calibration of the background sky will be given to those pictures, a needed step in the calibrating process of spectra.

★ A real scientific and technical challenge!

Working with a professional telescope in Chile does not mean either the TAROT experiment will work 100% perfectly. A relevant number of elements could disturb the ongoing measurements of light deflection, as well as natural or purely technical.
La Silla covered by snow, june 2013. Credit: ESO/M. Tewes
The first limiting factor which could cause a straight forward cancellation of the experiment is obviously the weather. In southern hemisphere, winter will have the upper hand on the atmospheric conditions around the area. Statistically, July is even the worst period of the year in-which snow and cloud coverage are bound to happen several times.

From an instrumental point of view, measuring variations of position of stars with gravity requires an extremely good accuracy. Astronomers want to detect a deviation angle of about an arcsecond or so ( = 0.00027 degrees!). This value represents a relative motion of stars of 0.33 pixels onto the CCD's sensor at prime focus of TAROT. Complex dedicated image processing pipelines have already been programmed by Alain Klotz. The whole principle of the measurement lies on calculating relative distances between stars located in the field. Ideally, a minimum number of 10 bright stars could lead to satisfying results.
A second source of mistake the astronomers are aware of is the field's misalignment done by the telescope's mount. Indeed, the image captured during the eclipse should contain exactly the same region of the sky as the reference image which has been obtained few months ago, prior to the Eclipse. These pointing errors are a big concern, but can be reduced using some tricky post-editing methods.
Even if a total solar eclipse makes the common daylight to fade out drastically, it does not rely exclusively on visual effects. For example, gently decreasing daylight is similar to simulating a pseudo-night during which temperature could drop down very fast. This 7°C - 10 °C brutal thermal variation triggers disturbing phenomena regarding the focus adjustment or the quality of images.

Finally the entire totality phase will be contained within less than 2 minutes: precisely 1 minute and 51 seconds. In 1919, astronomers were far luckier since they witnessed the longest total eclipse within a period of 5 centuries. Eddington and Dyson stared at it for 5 minutes!
Going back to 2019, this short duration will limit the number of frames taken by the french astronomers at La Silla. Any mistake will not be forgiven, as they could compromise the project in few seconds.

All this error sources should be handled with care by the team. What seems at first sight a simple 1919-double experiment turns out to be a bit more tricky, even 100 years after!

A so-called failure will brake this long-prepared project into pieces. Therefore, the amateur astronomy expert Christian Buil bent over backward to find an emergency plan. This plan-B consist in using an aprochromatic refractor equipped with another CCD camera, both installed on the same mount as TAROT's. This second setup will be controlled separately from the primary experiment in case of something bad happens.

★ Why conducting this experiment again?

Even though the 2019 experiment is the most honorable tribute to the 1919 experiment which shed light on the newly-born mad idea of Einstein, this was not the first reason of conducting it. TAROT experiment points out the fact of undeniable relation between both theoretical and experimental aspects of science. Beyond the shadow of a doubt, the 2019 experiment at La Silla will give to scientist a realistic overview of the main difficulties Eddington and Dyson encountered.
The French astronomers trio will not only publish their results to the scientific community but also share it to the general public, creating pedagogic supports and posters to the European Southern Observatory 's public outreach department.

Stay tuned of the final outcome of this experiment and follow AstroSpace or @AstroGuigeek to get live updates on social media.

Source credits: Remarks and words gathered during an interview of A.Klotz, C.Buil and J-F.Le Borgne carried out in Toulouse (France) by Guillaume Doyen on May 22nd 2019.

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@AstroGuigeek Ingénieur doctorant, Astronome et Photographe amateur / French Engineer & Ph.D student, Astrophotographer & Amateur Astronomer. I simply love sharing my experience, advice and facts on Astronomy.


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