The art of occultations: March 2026 meeting recap

By Dirk Goës

The March 2026 meeting was our first online Zoom meeting for the year.  We welcomed all existing and new members and acknowledged First Nations peoples.  SCS President Dr Toner Stevenson gave an overview of the results of our member survey.  The survey found that our members enjoy a mix of learning about amateur astronomy techniques as well as learning about what professional astronomers are up to in their research.  The survey results also strongly endorsed the format and length of our monthly meetings. 

SCS Secretary, Elizabeth Cocking, updated members on matters to do with the society.  Elizabeth read out club member Garry Dalrymple’s solar observation report for the months of January and February. 

Our first presentation was a video on Lunar Eclipses created by club member Robert Luxford.  It was a timely educational video with a total lunar eclipse occurring in Sydney on the following night.  You can watch the video again below.

https://www.youtube.com/watch?v=X8OudXll9wQ

Credit: Robert Luxford

Our keynote address was by Blue Mountains amateur astronomer Dave Gault.  Dave specialises in observing and recording occultations from his home observatory named ‘Kuriwa’.  Together with a small network of dedicated occultation observers Dave produces some impressive science results such as determining the size, shape and orbits of asteroids. 

An occultation occurs when an astronomical object such as a star is temporarily blocked from view by another object such as the Moon, the Sun or an asteroid. 

Credit: Dave Gault

Dave discussed Lunar occultations and showed two fascinating examples.  The first was a video of the double star Antares re-appearing from behind the Moon.  First the fainter Antares-B appeared and then the much brighter Antares-A appeared engulfing the fainter star.  The second video showed a lunar graze where a star disappears behind the limb of the Moon and then re-appears between surface features of the Moon. 

Dave described the science of asteroid occultations and how multiple observations of an asteroid occultation by astronomers in different locations can collated to calculate the shape and size of an asteroid. 

An example of how multiple occultation observations can be used to calculate the size and shape of an asteroid.  Each line (a chord) represents one observation with the astronomer’s name recorded in the legend (Credit: Dave Gault).

Dave explained how occultation observations proved invaluable in measuring the success of the NASA Double Asteroid Redirection Test (DART) in 2022. The DART mission was a test to see if a spacecraft could be used to deflect an asteroid.  The DART mission collided with Dimorphos, a minor-planet moon of the asteroid Didymos.  Occultation observations not only provided very accurate measurements of the change in orbit of Dimorphos but also showed that the orbit of the parent asteroid Didymos was altered, which was not picked up by other observation techniques. 

Australia has a very dedicated network of occultation observers with one-fifth of worldwide occultation observations being done in Australia.  Dave is looking to expand this network and would like to hear from amateur astronomers who are interested in participating.  Dave can provide advice on techniques, equipment and software.  If you are interested in being involved in some real science, please contact our President Dr Toner Stevenson or one of the SCS committee members and we can put you in touch with Date. 

Our next meeting Keynote address is titled 'Astrophotography'. This is on-line and presented by amateur astronomer, Grahame Kelaher on Wednesday 8 April, 6:30pm.