Mars goes behind the Moon – 17th April, 2021

Lunar occultation simulations courtesy Stellarium

On the evening of April 17, 2021 you can enjoy seeing an amazing event in the sky where a crescent Moon will eclipse (occult) the red planet Mars between 5:30pm and 7:30 pm Indian Time. This event will be visible from many parts of India, especially in the East. The disappearance will occur when it is still daylight i.e. around 5:30pm IST. This can be observed using a binocular or a telescope, but extra caution should be taken when the Sun is present in the sky. The reappearance will occur after sunset, during twilight after 7:00 pm IST, depending on where you are in India.

The exciting event is something you can make a reason to spend some time with the family under the evening sky and a crescent Moon. The ASI-POEC has created a resource base on this page for anyone to see and share the transit. Scroll below for Webcasts, Information and Suggestions to share.

Live Webcast

Watch live streaming video from IUCAA, Pune

Want to share the event?

If you have a small telescope, why not try and set up an observation and webcast? Click here to know more.

What will we see?

What we expect to see is the red planet Mars going behind the Moon and then reappearing in about two hours. At the time of this event (also called an occultation), the Moon will be 5 days past the new moon and appear as a 24% illuminated crescent. Mars will disappear behind the unilluminated side of the Moon and reappear from behind the illuminated part of the crescent.

As explained here, the disappearance of Mars may not be visible for many places in India because it happens when the Sun is up or just setting. But, you may still take the challenge of catching it with your binoculars or telescope (see this warning). Just find out the approximate time for your location and be ready to catch Mars blip out behind the invisible edge of the Moon's crescent. You should see a fading in the Mars’ light before IMMERSION (the term used for the disappearance in an occultation). Since Mars is seen as a small disc, it will take a few seconds for the planet to disappear or reappear from behind the Moon. This is best seen with a telescope.

The next exciting part will be knowing exactly where and when the EMERSION of planet Mars (as the reappearance is called) & catching it peek out. This bit can be challenging as you cannot see the planet and it comes out from somewhere on the Moon's bright edge. Given that the Moon's apparent size is about 450 times the planet on this day, it is necessary to estimate where it will be seen during the few seconds of reappearance. With a good clock, you may time both the events as a useful exercise.

While these "events" last for a few moments, the fun part will be the waiting period before Mars reappears. Twilight wil be setting in and you may see the bright stars slowly appear as the sky turns dark. Go ahead and identify some of them and the famous constellations close by like Orion (Mruga), Taurus (Vrishabh), Auriga etc. If you have a small telescope (even a 3-4 inch diameter one) you could take a peek at the lunar surface and try and identify some features on it. It is breathtaking even just to gaze on it and you may feel like you are flying to the Moon.

Using Telescopes and Binoculars

This is an event that can be part enjoyed with the naked eyes, but would be better if you have a pair of binoculars, or better still, a telescope. The Moon moves across the sky, much faster than we see Mars move. You may remember that this is similar to a Solar eclipse where you see the Moon moving across the Sun's face. But in that case the Sun appears to be almost the same size as the Moon. Here, Mars will be about 450 times smaller than the Moon. Please note that these size differences are all apparent and not the real sizes.

Warning: Keep your binoculars or telescopes away from the Sun! Extreme caution is necessary when pointing at the sky with the Sun still above the horizon. Even by mistake if there is a momentary glance at the Sun through such an instrument, it can cause permanent damage to your eye.

What to note and wonder?

  • Can you figure out the rate at which the sky moves relative to you? Why does it move?
  • Can you figure out which direction the Moon moves around the Earth? Are there two apparent motions in play here?
  • Can you figure our the (angular) speed of the Moon relative to the rest of the sky?

Details and timings at some Indian locations

The Sequence will be somewhat like this >>

1st contact: start of disappearance of Mars from East of Moon → 2nd Contact: Complete Occultation → 3rd contact: Start of reappearance at Western Limb → 4th contact: End of Occultation

(Details provided by Arvind Paranjpye. Please see and feel free to use the images below)

Table with timings for some representative places in India. Please adjust your observations according to the nearby location.

Table with timings for some representative places in India. Please adjust your observations according to the nearby location. See details in the text alongside or consult this PDF file.

Paths of Mars according to location numbers in the above table. It will move East (disappearance point) to West (reappearance point).

In India, observers in the far east will be bale to catch both the events described above. As we move west the Sun will be higher in the sky and may not even have set when the first contact occurs.

See the table alongside. It mentions the Sun's altitude (angle in degrees from West Horizon) at the time of the 1st and 3rd contacts. A negative number means that it will have set and be at that many degrees below the horizon (a blank means it is practically night). This will help you plan your observation.

The table also mentions the corresponding altitude (or angular "distance") of the Moon. Note that the Moon is approximately 58° away and hence it should not be too difficult and unsafe to point your scopes to it even during daylight.

Mars will not pass right behind the centre of the Moon at all places. The diagram below shows it's apparent path (from East to West) numbered corresponding to the location number mentioned in the table.

More details

* Table of disappearance and reappearance of Mars for about a hundred locations in India by Arvind Paranjpye

* Local circumstances for the whole region by the International Occultation Timing Association

Some Observational details:

Mars (mag 1.5) will be occulted by the Moon ().
The elongation of Mars will be 58.7°.
Distance to Mars will be about (a ray of light will take 15m 52s to travel from Mars to Earth).
Angular diameter of Mars will be 4.90" (arc-seconds), with a phase of 0.92.

Since Mars appears only 1/450 of the Moon's apparent diameter that evening, a telescope with a magnification of 50x or more is better to watch this event.




Here are some informative webpages :




What is an Occultation?

An occultation is an event when a closer object passes in between the observer and a farther object. The farther object is either partially or fully hidden(occulted) due to the closer object.


Sharing the Transit

The ASI-POEC calls for sharing this event by telescope owners and science popularisers across the country. At their own locations volunteers could arrange to show this through live webcasts so that many more people got to see the occultation at least online, overcoming reasons like daylight, bad weather etc.

If you have a small telescope, and have access to a location from where the western sky can be seen clearly, try and set up an observation and webcast. Join us in a nationwide effort to share this spectacle with all people. Please see the videos below and contact for help required if any.


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