South Plains Astronomy Club

Observing Under The Dark West Texas Skies

Betelgeuse is a variable red supergiant star that is the shoulder of the constellation Orion, visible in our night sky during winter. The star’s name in Arabic is “Yad al-Jawza”1, meaning “The hand of al-Jawza” (Jawza is the Arabic name for Orion.) and was mistranslated in the 13th Century. In English, the name is pronounced in four ways, often sounding like a variation of “beetle juice“, like the movie, or “bet el jooz.”

Orion constellation map2

A Variable Supergiant Star

It is usually the 10th brightest star in the night sky, though it does vary in brightness. If it replaced the Sun, it would extend all the way out to the asteroid belt, which is 2-3 time farther from the Sun than Earth! It has 10-20 times the mass of the Sun and is estimated to be about 500-600 light years from the Sun, which is a large uncertainty for a close star. At less than 10 million years old, this massive star is expected to last only 100,00 years longer before exploding as a supernova that will be about as bright as the half Moon for three months!

It’s close enough and large enough that astronomers actually measured its angular size in 1920. The angular size has varied between 0.042 to 0.056 arcseconds, which is about 25%! The star is believed to pulse in size, be non-spherical, and also be variable in brightness.

Note the shape and size of the dim outer envelope near the box drawn in the image and irregular shape of the center brightness.3

The Great Dimming

In the fall of 2019, Betelgeuse dimmed so much it was visibly changed to the naked eye! By January 2020, the star dimmed two and a half times from magnitude 0.5 to 1.614, which is the lowest magnitude of the star in the 25 years of the study. (Higher numbers are dimmer.)

Betelgeuse light curve by AAVSO observers. The gaps are times when the star is not visible in the night sky.4
This comparison image shows the star Betelgeuse before and after its unprecedented dimming. The observations, taken with the SPHERE instrument on ESO’s Very Large Telescope in January and December 2019, show how much the star has faded and how its apparent shape has changed.5

Of course, the mainstream media jumped on the theme that the star was about to go supernova, but that is unlikely for tens of thousands of years. In August 2020, the Hubble Space Telescope completed a long-term study of the event and the ultraviolet observations showed that the dimming was probably caused by a massive ejection of material towards Earth that cooled and blocked approximately 1/4 of the star’s surface.

Illustration of the Betelgeuse mass ejection that caused the dimming.6

Several other astronomy groups confirmed the dust theory by June 2021. In August 2021, a second dust cloud caused another significant dimming. The dimming could have come from a short-term 416-day cycle minimum coinciding with the long-term 2010-day cycle minimum to produce a grand minimum that is visible in the light curve from the deep dip to the gradual brightening back to magnitude 0. By April 2023, Betelgeuse reached its normal peak of 0.0 magnitude.

Naturally, scientists need further observations of Betelgeuse to know more about the processes on the star. Fortunately, on Monday, December 11th, we have an occultation that provides that opportunity.

What is an Occultation?

An occultation is a solar system body such as an asteroid, planet, our Moon, a planetary moon or even a comet occulting or eclipsing a star. These alignments are rather magical to me. The star is tens or hundreds of light-years away as a point source (Betelgeuse is rare exception – More on that later). Asteroids, moons, and comets are usually light-hours away and are tiny disks. You usually can’t see the solar system body, so visually, an occultation is a star blinking out for less than a second to tens of seconds while the stars around it stay where they are. The light from the star is effectively parallel, so the shadow and the path are the size and shape of the asteroid.

I volunteer with International Occultation Timing Association (IOTA), which is a collaboration between amateur astronomers and professional astronomers. The occultations occur across the globe, often far away from professional observatories. An observatory is rarely in an occultation path and can’t be diverted for such an event. Therefore, they depend on amateurs to use common telescopes with GPS and video systems to time the length of an occultation.

The modern magic comes in when you look at the precision and math that it takes to find a shadow path on the Earth:

  • The point-source star has a slow proper motion. (precision in position and motion)
  • The star may have a companion (precision in orbit or wobble of either star)
  • The small disk of an asteroid is in orbit around the Sun and rotating (precision in size, orbit, shape, and possibly rotating)
  • The asteroid may have a companion (precision in wobble of either body, orbit, size, shape of companion)
  • The Earth is in orbit around the Sun (precision in location)
  • Observer is rotating with the Earth (precision in rotation, location, and altitude)
  • Time – To get an accurate prediction, the position and motion of each piece must be known accurately.
Geometry of an occultation7

IOTA Predictions for Betelgeuse

Fortunately, IOTA has software that generates predictions with as much precision as existing data provides. Observatories can make astrometric observations of the star and/or asteroid for high-interest, but low-precision events. Once the predictions are made, the software produces an overview map of the path against the globe. The path prediction for the Dec 11, 2023 occultation of Betelgeuse by the asteroid (319) Leona looks like this:

IOTA Path Prediction of Betelgeuse-(319) Leona occultation8

It includes information on the event, star, and asteroid. The duration of this event is 10.6 seconds at the centerline and will sweep across the earth from 01:08 to 01:26 UT tonight. Betelgeuse has a predicted magnitude of 0.5 while the asteroid is just 14.2. The asteroid is 50km with an error of ±3km, which defines the shadow path and 1 sigma lines.

The shadow path of the event stretches from Mexico to Turkey and IOTA members receive notifications via Occult Watcher software. Members along the path or willing to travel volunteer to observe this event and pick a spot along the path. (See the images below.)

Dozens of amateur astronomers from Florida to Azerbaijan have volunteered for this high-interest campaign:

IOTA software map to select a location for observing the
Dec 11, 2023 occultation of Betelgeuse by (319) Leona.9

In the path maps, the colors represent various limits in the event:

  • Green – Predicted center line of the path
  • Yellows – Observer’s positions from the centerline
  • Blue – Predicted edge of shadow path
  • Red – 1 sigma limit (error margin for the event, which is large in this case)
  • Telescope – observers’ positions
Betelgeuse Occultation Florida Map, Dec 11, 2023
Florida Occultation Map,
Dec 11, 2023 occultation of Betelgeuse by (319) Leona.9
Europe Occultation Map
Dec 11, 2023 occultation of Betelgeuse by (319) Leona.9

Recording the Occultation

In the past, IOTA members used a shortwave radio tuned to WWV, the atomic clock, a telescope, and a voice recorder. We would watch the star and call out when it blinks out and comes back on again. You can get surprising accuracy using this method.

Today, the telescope remains the same, but now we have a video system with a time overlay. The clock is taken from the GPS system so observations can be synchronized. The overlay system puts the clock, location, and other data into the video feed, which means each frame has a different time stamp. The combination gives you accuracy to the video frame, 1/24th of a second or faster.

Example of a test of the GPS timing system using an LED (white square)12

Depending on the magnitude of the star and asteroid, IOTA members use telescopes from 50mm up to 14″ telescopes. Many members have built special purpose telescopes that are easier to transport on an airplane or in a car.

One of the smallest occultation systems is a small security camera paired with one half of a 50mm binocular.13
A 10″ collapsable telescope on a hand-made pre-point mount.14
This is my take on the occultation scope using a 10″ f/4.8 mirror in a menudo pot that can mount to my alt/az or equatorial mounts. My recording gear sits in the briefcase, powered by a small 12v battery.

Without a tracking mount, we pre-point at a known star at a time that will allow the telescope rotate in time to view the event. The cameras range from low-light drone cameras that I use to more expensive astrophotography cameras. IOTA offers the timers for sale.

The end result is a video starring a star that disappears:

461 Saskia occulting TYC 1324-01960-1, March 18, 201715

After the Event

Each observer uses another software package (one of several) to create a data set of the target star’s brightness. This creates a light curve that shows the light drop of the target star. The newest software monitors the brightness of another star in the field to track variations in brightness due to haze, thermals, and other disturbances.

Sample Limovie output for the (461) Saskia March 18, 2017 event.
If you’re lucky, you get a strong, clear drop in the data like this one.16

Each observer then submits the data generated by the software to IOTA. Then another software package is used to analyze the submitted data and reduces the data as though all observations occurred at the same time instead of spread across time and distance. Then the software places each observation at the correct location from the centerline, forming a virtual picket fence across the path. In the case of tonight’s occultation, that works out to an observer every mile or two across the 50 miles of path.

Once the observations are reduced to the same time and set at the proper distance, the software assembles the observation to show the asteroid profile:

Profile of double asteroid (90) Antiope
2011 July 19th occultation of LQ
Aquarii by Antiope by IOTA observers17

What we hope to find

When combined with other observations of that occultation or other events, produces a surprisingly large amount of data, including:

  • Position and possibly size/atmosphere of a star
  • Position and orbit of a companion star
  • Existence of a tight companion star that cannot be observed with telescopes
  • Size, shape, and orbit of an asteroid
  • Size, shape, and orbit of an asteroid’s companion
  • Depth of atmosphere in a planetary moon

Since Betelgeuse is large enough and close enough to have an angular size, this occultation is a great opportunity for scientists. It’s hoped that the occultation can show variations in the size and brightness of the star’s envelope to a higher resolution than is possible in observations from professional observatories.

I will post a second article once the results are released, but you can check for yourself at https://call4obs.iota-es.de/news-feed-occultation-of-betelgeuse-by-319-leona. That page will also feature live feeds of the event.


  1. Wikipedia – https://en.wikipedia.org/wiki/Betelgeuse#Nomenclature ↩︎
  2. By Orion_constellation_map.png: Torsten Brongerderivative work: Kxx (talk) – Orion_constellation_map.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10850823 ↩︎
  3. By Alex Lobel, Andrea Dupree, Ronald Gilliland, CfA, STScI, NASA, ESA – https://www.cfa.harvard.edu/news/archive/alobelimg.html , Public Domain, https://commons.wikimedia.org/w/index.php?curid=5829202 ↩︎
  4. AAVSO – AAVSO Light Curve Generator 2 (LCG2): https://www.aavso.org/LCGv2/ ↩︎
  5. ESO/M. Montargès et al. – https://www.eso.org/public/images/eso2003c/ ↩︎
  6. NASAESA, and E. Wheatley (STScI) https://hubblesite.org/contents/media/images/2020/44/4725-Image?news=true ↩︎
  7. IOTA – https://occultations.org/occultations/what-is-an-occultation/ ↩︎
  8. IOTA, European Section – https://www.iota-es.de/betelgeuse2023.html ↩︎
  9. IOTA – OccultWatcher Cloud https://cloud.occultwatcher.net ↩︎
  10. IOTA – OccultWatcher Cloud https://cloud.occultwatcher.net ↩︎
  11. IOTA – OccultWatcher Cloud https://cloud.occultwatcher.net ↩︎
  12. Bob Anderson, Test Report – https://occultations.org/documents/RunCam-Night-Eagle-Astro-test-report.pdf ↩︎
  13. Scotty Degenhardt – https://scottysmightymini.com/ ↩︎
  14. Richard Nugent – https://www.poyntsource.com/Richard/IOTAMeeting2011.htm ↩︎
  15. Derek C. Breit – https://www.poyntsource.com/New/Archive/SaskiaResults.htm ↩︎
  16. Derek C. Breit – https://www.poyntsource.com/New/Archive/SaskiaResults.htm ↩︎
  17. IOTA, Plotting accurate paths for the (90) Antiope occultations https://occultations.org/publications/rasc/2022/PlottingPathsForAntiopeOccultations.pdf ↩︎