Measuring the solar radius during the 2023 annular-total solar eclipse

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About This Project

Total solar eclipses are unique opportunities to measure the value of the solar radius, a fundamental astronomical quantity, and to investigate its variability through time. Those moments when the Moon is about to fully cover the Sun provide an unparalleled view of the light emitted by the lowermost layers of the solar atmosphere, a light hidden by the Sun's glare at all other time. By analysing its spectrum (the flash spectrum), the solar radius can be estimated.

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What is the context of this research?

The radius is one of the fundamental quantities describing the shape of our star. Being the Sun a gaseous body, various definitions of solar radius exist depending on the context. If we focus on the "complete photospheric extinction" radius, as measured during total solar eclipses, its value has been shown to be higher (ca. 960") than the canonical value of 959.63" used for over a century in eclipse computations and many other fields. Past studies have also hinted at the tantalising hypothesis that the solar radius might fluctuate over time. Improving the measurement of the solar radius and performing it at every total solar eclipse allows refining our knowledge of this fundamental quantity and monitoring its variability through time.

What is the significance of this project?

We collected flash spectrum data during the 2017 total solar eclipse and we successfully estimated the value of the eclipse solar radius. We aim to improve the experimental technique we designed and to apply it during the 2023 annular-total eclipse. These eclipses are rather infrequent (next one is in 2031) but, due to their intrinsic short duration of totality, they present favourable conditions to record detailed flash spectra. The apparent lunar radius will be just 1% larger than the apparent solar radius and the emission arcs in the flash spectrum will be unusually long. A direct benefit of accurately estimating the solar radius is to lead to more accurate eclipse predictions.

What are the goals of the project?

We will collect a high-resolution time-stamped video of the eclipse flash spectrum during the 2023 annular-total solar eclipse, visible on land from Exmouth Peninsula (WA, Australia). We will observe from a location very close to the edge of the path of totality, so as to increase the sensitivity of the measurement. We will then estimate the value of the solar radius by extracting time-stamped light curves from the video and by comparing them to simulated light curves. The simulations will be based on a very precise celestial mechanical model of the mutual movement of the solar and lunar limbs. Possible extra goals: 1) deconvolving the limb darkening function from the observed light curves by reversing the simulation procedure 2) assessing the accuracy of eclipse models.


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The central task required for the experiment to succeed is the recording of a high resolution time-stamped video of the eclipse flash spectrum. The main piece of equipment to accomplish such a task is a full frame RAW video camera. The full frame sensor allows capturing the whole extent of the flash spectrum. The ability to shoot RAW videos provides access to the uncompressed sensor data. This is crucially important in order to be able to cleanly perform photometry and extract light curves from the flash spectrum video. Time-stamping capabilities for this camera will be provided by an external time-inserter that we will design. The CCD camera has a small sensor and only allows imaging part of the high resolution flash spectrum. It allows recording high frame-rate RAW videos and it has an integrated GPS providing accurate time-stamping of each frame. Using both cameras will provide redundancy and increased fault tolerance. Diffraction gratings are needed for spectrography.

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We live in an era when extremely precise predictions of solar eclipses have become possible. A decade ago, the Japanese Kaguya and NASA LRO lunar orbiters gave us our first high-resolution elevation model of the Moon, allowing precise geometric determination of the lunar limb against the Sun. The remaining largest uncertainty is the precise diameter of the Sun as measured from 1 AU. As an eclipse cartographer, I await the results of this experiment to give us confidence towards establishing sub-second accuracy in eclipse predictions and maps.

Flag iconProject Timeline

The project is centred around the annular-total solar eclipse occurring on April 20th, 2023. First, we will design and assemble the set-up needed to record time-stamped videos of the solar flash spectrum. We will then thoroughly practise the detailed step-by-step procedure to be followed on eclipse day. Eclipse are one-shot experiments: there is no room for mistakes! Finally, an estimate of the solar radius will be obtained by analysing the videos and performing numerical simulations.

Dec 14, 2021

Project Launched

Aug 01, 2022

Design and build a time-inserter to add time-stamping capabilities to the full frame RAW camera.

Jan 01, 2023

Assemble and test the optical equipment together with the time inserter. Design and practise the step-by-step experimental procedure to be followed on eclipse day.

Apr 20, 2023

It is Eclipse Day! Travel to Exmouth Peninsula (WA, Australia) and collect time-stamped videos of the flash spectrum of the solar eclipse.

Aug 01, 2023

Analyse the videos and estimate the value of the solar radius. If possible, deconvolve the limb darkening function / assess the accuracy of eclipse computational models.

Meet the Team

Luca Quaglia (Besselian Elements Team)
Luca Quaglia (Besselian Elements Team)
Alessandro Pessi (Besselian Elements Team)
Alessandro Pessi (Besselian Elements Team)

Team Bio

Besselian Elements Team: Luca Quaglia, Alessandro Pessi and Kostas Emmanouilidis are a team of dedicated amateur astronomers, passionate about solar eclipses. Besides being awe-inspiring phenomena to observe, solar eclipses provide the opportunity to do hardcore science. We are proud to strive to give our small scientific contribution to the great human endevour of expanding and deepening our knowledge of the universe that surrounds us.

Luca Quaglia (Besselian Elements Team)

I saw my first solar eclipse, a partial one, in 1994 from Northern Italy. While reading an article about that eclipse in an astronomy magazine, I became aware of the fact that a total solar eclipse would cross Europe on August 11th, 1999. I was on a hilltop in beautiful Alsace that day and despite being clouded out, the arrival of the lunar shadow was so impressive and so real that I was hooked! I told myself that I would go wherever next total solar eclipse would occur. I was a university student with little money but I scraped up enough to go on a special budget trip to Zambia and on June 21st, 2001 I saw an incredibly beautiful total solar eclipse in clear sky. My life was changed.

I wanted more, as I was in awe of people performing eclipse computations and predicting their circumstances. I started studying and I gathered enough knowledge to be able to make precise predictions. I then computed a very long canon of solar eclipses: basically a long list of so-called Besselian elements, spanning the time interval from the year -13000 to the year +17000. For about ten years that was the longest available eclipse canon. While performing those computations, I became curious about experimentally checking the accuracy of the predicted solar eclipse circumstances, especially the accuracy of the time of second and third contact. To pursue that goal, I came up with the idea of using the flash spectrum (I later discovered that others had had similar ideas in the past). And I also learnt that similar techniques could be applied to the estimation of the solar radius.

I am currently living in Sydney (Australia).

Alessandro Pessi (Besselian Elements Team)

Passionate about Solar Eclipses since 1999 after a first attempt under the rain in Germany. After that I had more luck in Spain, UK, Russia, Turkey, China (kind of), Australia and finally USA in 2017. I like the preparation phase, the planning, the adrenaline building up the days before and finally the mix of fear and awe that you fill in the dark under the umbra of the moon.

Being a maker and a data scientist, I receintly got involved with the HelioMetrics Team and its challenge and I'm really excited to prove that amaterus can make good computation and experiments and give a real contribution to science starting purely from their passion.

I'm currently living in Milan (Italy)

Additional Information

A thorough description of the experimental procedure and of the results of its application during the 2017 total solar eclipse can be found in: Luca Quaglia et al 2021 ApJS 256 36. Details on assessing the accuracy of eclipse computational models are described in Appendix C of the same paper. The methodology to deconvolve the limb darkening function from light curves is given in IJMPS 12 405.

We will publish updates also on the website:

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