Update – Occultation of Venus by the Moon on 24 March 2023
The joy of Occultation observations:
I am watching a star through the eyepiece of my telescope. The star is not a very bright one, but it is not too faint either that I have to strain my eyes to keep looking at it. Once in a while, the star slightly drifts, and I used the hand pad of the telescope to get the star in the centre of the field. And then all of a sudden, without any warning, the star is gone – it is just gone. It is no more there in the eyepiece.
This was not a chance event. The event was well calculated and predicted to an accuracy of better than a couple of seconds. I had been expecting what I observed; I have observed this kind of phenomenon a number of times in past and yet that sudden disappearance of the star and still adrenaline rushes through my body.
Sometimes reverse has happened too. Where there was nothing to see in the eyepiece for the last minute or so and a star just pops up in the field.
I have experienced such events countless times, mainly because I have not kept a log of all the events I observed. Anyway, even after so many years the experience vanishing stars or simply appearing in the eyepiece, observing a new event always mesmerizes me.
These events were OCCULTATIONS of stars by the Moon. Wikipedia describes an occultation as an event that occurs when one object is hidden by another object that passes between it and the observer.
To get a feel of what one sees during the observation of the occultation of a star check my YouTube video <click here>. You will hear time beeps every second. There were from the GPS time signal app.
In the early days, prior to GPS, we also recorded time signal beeps. These beeps are transmitted by a radio station every second at 10MHz. To indicate the one full minute, a long beep is transmitted. By recording these signals along with the occultation event, one can find the exact time at which the event took place.
There are two types of events associated with occultation: disappearance and reappearance. In the case of the Moon, events also have two variations – events taking place at the darker or brighter limb of the Moon. The example above was the disappearance of a star Omega 1 Tauri on 17 March 2013. This event was recorded from Nehru Planetarium, Nehru Centre, Mumbai.
Moon also occults planets. Unlike stars disappearance or reapperance of planets is gradual. The link to the two videos below is the disappearance and reappearance of Mars.
The disappearance of Mars behind the Moon and its reappearance on March 10, 2008.
In the disappearance event, you can make out the curved lunar profile. And you can also make out the difficulty in capturing the reappearance event.
[These videos were made using a web camera. The lens of the camera was removed, and an adapter was made so that the camera replaces the eyepiece of the telescope.]
Such is a joy of occultation observations. But occultations are not just for visual pleasure. From time to time observations of occultations have played a major role in understanding the nature of various elements of our universe.
Arvind Paranjpye, email paranjpye.arvind@gmail.com
Occultations 