Observing Lunar Occultations

Recording Occultation timings (visually)

You can make some contribution to the science of astronomy by accurately timing disappearance of the star behind the lunar disk and/or it’s reappearance from behind the lunar disk and noting down the exact location from where you have taken these observations. 

During the pre-smartphone era, these values were not so easy to obtain but now it is just too simple. To time the event you will need at least two smartphones. One that has a GPS time clock and one to be used as a stopwatch. 

Getting coordinates of location

You can use http://wikimapia.org/ to get exact coordinates of the location of your observing station.  The image below is a screen capture of the area around Nehru Centre, Mumbai (at bottom right) and Nehru Planetarium which is marked as hexagonal building at top right.  The ‘+’ mark between the two is where the telescope was located.  The coordinates of the telescope can be read out at the bottom left of the wikimapia screen (not shown here). 


Using the map the location of our observing station read as

Latitude  18 deg 59 min 21.3s N and Longitude 72d 48m 55.4s E

Most phones do have GPS and when you click a picture the coordinates of the location are automatically stored in the metadata file of the image.  You may have to enable the ‘location’ tab.

Getting the correct time:
The smartphones have a good stopwatch. But it helps to install GPS time app. Some of these apps can actually correct time of your smartphone.

Update : (7 Jul 2020)
Alok Mandavgane an avid amateur astronomer and astro-app maker has made a very useful app for android phones, ‘Sat Timer’. This app solves the issue of getting the event time (see below) and coordinates of the observer. The app takes time and coordinates from GPS satellites. Click of a button will give you the time of the event and coordinates of the location. And since you can click many times this app is useful in case you clicked by mistake before the event. And most useful in grazing events.

The app also has set countdown. The purpose of this utility is to remind you time remaining for the event.

Experience tells us that never set this time closer to the event. In earlier days when we used to do group observations, the time keeper will announce the time of the event five, four, three, two and then ‘one minute to go’. After that there is no more announcement made. It is then up to the observer to time the event.

I strongly encourage you to download and install this app.

Right screen captures show the time in UTC along with some (dummy) captured data. One could also set the local time. The alt: = altitude of the observer is not correct and is to be ignored.

Occultation observations:
Lunar occultations are very easy to observe as you do not have to have any star map to search for the star as long as you can see the Moon in the sky. In the case of disappearance event, an hour before the predicted time of the occultation, the star to be occultated will be just about quarter degree from the lunar limb.

  1. The disappearance of a star at the darker limb of the Moon. This happens between the new moon and full moon.  These events are simple to observe. Though closer to the full moon time the observations can be become very difficult due to brightness of the Moon. All you need to do is to point your telescope in the direction of the Moon and move it bit to east and you are sure to find the star to be occultated.
  2. The disappearance of a star at the brighter limb of the Moon. This happens between the full moon and the new moon phase.  The difficulty level depends on the phase of the moon and the magnitude of the star. Again you locate the Moon and move the telescope to east.
  3. The reappearance of the star at the darker limb of the Moon. This happens between the full moon and a new moon. Generally timing the reappearance is always a bit too difficult. You have to point your telescope accurately so that when the star reappears it will be in the field of the eyepiece. This can be tricky.
  4. The reappearance of the star at the brighter limb of the Moon. This happens between the new moon and the full moon. This event is relatively easy to time as one can have a moon map to accurately position one’s telescope. Of course, again it depends on the brightness of the lunar limb and the magnitude of the stars.
  5. Lastly, we have grazing occultations – to observe these you have to locate your self at the northern or southern limit of the occultation track.  These are great fun to observe. One sees the star blinking as the star is seen through valleys of the Moon.  This is quite similar to Baily’s beads during the total solar eclipse.

Note: Occulations events taking place a day or two before and after to new moon and full moon phase are generally very difficult to observe.

What happens at the telescope

In the case of the disappearance event, you are watching the star through the eyepiece of your telescope. Depending upon the weather condition and the magnitude of the star it might be blinking or shimmering continuously.  Up to the first quarter of the moon, you may also notice faint dark lunar limb slowly approaching the star. But after first quarter of the moon you only see the star in the field of your eyepiece.

Closer to the event adrenaline rushes. The excitement is at its highest.  And then in a jiffy, the stars blink out the field – just gone. You have to time this event.

If the event is taking at the brighter limb of the Moon and if the limb is equal or brighter than the star then it becomes quite difficult to time the event. 

In the case of reappearance – the star appears to just pop-up from nowhere. For last one minute or so there was nothing in the field of view and now there is a star. It is a wow! feeling.

And since it can be difficult to accurately position the telescope the star may pop-up anywhere in the field. You will have to be very vigilant to time this event.

If you are not quite happy my using ‘just pop-up’ observe one event and you will know what I mean.

Timing the event:
This discussion is not quite useful in view of Alok’s app nevertheless do read on.

What I do is to keep my stopwatch ready and start the moment I see the star disappear or reappear.  Then at some fixed time as shown in the smartphone with a GPS clock, I stop the stopwatch or click on the lap time.  

The difference between the two will give the time of occultation.

Here is an example:-

A – The stopwatch has started the instant of the event.

B – the stopwatch was stopped when the GPS time showed  22:28:00

C – Time in the stopwatch   00:02:08.37

The time of occultation = B – C 

                                     = (22:28:00 – 00:01:02:37)

                                     = 22:27:57.63

Simple isn’t it.  Then go for it.

About skytonight

I am the present Director of Nehru Planetarium of Nehru Centre Mumbai, India I like to talk about astronomy and sky observations to general public.
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