Supermoons, that unlikely event in our routine life which kicks out a storm whenever it appears. All of us are quite excited about it. This is an important time for astronomers, stargazers, and astrophotographers. It is our refuge from the mundane world giving us an opportunity to look up into the sky.
Now, the question is what exactly is a supermoon?
A supermoon occurs when a full moon comes closest to the earth and appears 14% larger than what we see in ordinary. It’s called the Perigee or Perigean moon as the closest position to earth is termed as the perigee position. On the other hand, the time when the moon is farthest from the earth it’s termed as the apogee position and the moon is called the apogee moon. All of this is due to the elliptical orbit of the moon. But all such perigee positions doesn’t give rise to a supermoon. Thus supermoons are a rare event and occur once in a while in a year when extreme perigee coincides with a full moon.
So, at certain times of the year, you get to see a supermoon when the moon shines brighter than usual and appears 14% larger than its perceived size. 2 things are important here
- The sun’s and the earth’s gravitational pull which changes the shape of the moon’s orbit making an extreme perigee.
- A full moon occurring at the time of an extreme perigee.
The name supermoon which we hear today was actually given by Astrologer Richard Nolle (in 1979) due to its unusually bigger size. Here’s a tentative schedule of the supermoon as predicted by Nolle.
According to this list, there are 2 supermoons occurring in the year 2018, on 1st and 31st January. So, the year starts with a supermoon month and also a lunar and blue moon in January. The eclipse is scheduled in the 2nd full moon of the month, on 31st January. 2 full moons in one month makes it a blue moon year with the 2nd full moon being the Blue Moon. So, the year begins with a bang.
Click on the link to see the full list.
How supermoons occur?
Similar to the eclipses, supermoons occur in pairs, that is, there is a propensity of 2 supermoons occurring consecutively in the upcoming months. So a supermoon in December is followed by one in January. As it happened this year. Astronomers use a graph that predicts the occurrence of supermoons based on tehir relative distance from the earth. The supermoons are marked dots appearing at the bottom of the graph.
The equation used for this graphical analysis is as follows,
Relative Distance = (Da – Dfm)/(Da – Dp)
Now, this relative distance is the distance of the full moon (Dfm) from apogee (Da) and perigee (Dp) position. When this relative distance is 1, it’s in the perigee position and when it is 0, it’s in the apogee position. A relative distance of 0.9 marks a supermoon position. This explains the unusually bigger structure.
The same equation provides insight into the relative brightness of the supermoon. In this case, the comparison is between the apogee and perigee position. “A supermoon is typically 1.3 times (or 30%) brighter than a Full Moon at apogee, and 1.15 times (or 15%) brighter than a Full Moon at the Moon’s mean distance.” So this explains the bright supermoons that you see.
When is the ideal time to view a supermoon?
A supermoon is a perigee supermoon which is brighter and bigger than the usual moon seen on regular days. However, the larger size can’t be perceived by a layman that easily. A trained professional like astronomers might be able to recognize it instantly but others can’t. One sure way to catch the change is to view when it rises just after the sunset. A rising supermoon at the horizon is the best way to view a supermoon. Another way, to get an idea about the size is by comparing it beside some tall high rising buildings or monuments.
The above picture shows the November 13th supermoon of 2016. See, that the picture on the right seems bigger than that on the left. This is due to the time of the picture taken. The right-hand side picture was taken when the moon was on the horizon and the left-hand side picture was taken when the moon was high up in the sky!
Now, that you are equipped with information, go get a look at the supermoon tonight. Catch them on the horizon! For more information on the trajectory of moon this year, visit this page https://moon.nasa.gov/resources/154/