Supermoons happen when a full moon approximately coincides with the moon's perigee, or a point in its orbit at which it is closest to Earth.
As lunar eclipses generally occur in the middle of the night, not many would bother to wake up and see the beauty. They are content to see the photograph next morning in the paper or online. However, they do not realise that they have missed a mesmerising sight in the sky, which has inspired many poets in the past. Some have looked at it as a metaphor for melancholy, while others have seen it as a sign of detachment and some found it romantic too. The moon can be orange, brown or simply dark and completely invisible. The colour of the eclipsed moon is measured in Danjon scale devised by the French astronomer Andre Danjon. The darkest is numbered zero and the brightest as four.
The word 'supermoon' is of a recent origin and was first used in a 1979 magazine article to attract the attention of otherwise reluctant readers. But it wasn't until the last few years that the term received more attention. As a consequence of the varying distance from earth due to the elliptical orbit of the moon, it appears larger on a specific day of the month and smaller approximately 15 days later. When this day of proximity (which is known as the perigee) of the moon happens to be a full moon, quite naturally it has to appear larger than the usual size. But how large? Can you make out by just looking at it - that it is larger than the full moon you saw last month? Thus, for a common person, this has no relevance.
Closer to the sun
It is time to recall an instance that happened 19 years ago - December 22, 1999. It was a full moon at perigee. The moon was termed the largest and the brightest because earth (and therefore, the moon) was at the closest distance to the sun. The variation in the distance was barely 2%. However, the hype went to the extreme of declaring that Boston in USA was closer to the moon than Washington!
The earth is closest to the sun on January 3 or 4 every year. It is farthest on July 3 or 4. Let us take a look at the distances of full moons this year. We know that full moon is an instant and not a day. Let us also check how far apart the perigee and full moon instants were. On January 3, the moon was closest to earth at a distance of 3,56,566 km and 4.5 hours after the full moon. On December 3, 2017, it was 3,60,064 km and the full moon occurred 17 hours before the perigee. The full moon on January 31, which is also declared as 'supermoon', occurs 27 hours after the moon has passed closest to earth at 3,59,000 km. For records, one may write that there were three successive 'supermoons' because these differences of a few thousands of kilometres hardly matter.
Compare this with the full moon that occurred on December 22, 1999. The full moon occurred just five hours after the closest passage at 3,56,700 km. The full moon in November 1999 occurred just four hours prior to the close passage at 3,57,000 km.
We can also recall the eclipse of January 21, 2000. The full moon and perigee (3,59,400 km) were separated by just six hours. Now, exactly after 18 years and 10 days we have an eclipse. This cycle of 18 years, 10 days, 8 hours is called a Saros cycle.
The eclipse of January 31 is considered special since it is happening when the moon is at perigee. For us in India, it is visible at moonrise. The second eclipse of the year is later in July. It happens just two hours after the moon has passed the farthest point, called apogee. Therefore, that full moon will be the smallest and is known as a 'micromoon'. As this total lunar eclipse will happen during the second full moon of this month, the term 'blue moon' is used. However, 'blue moon' is a misnomer. There is nothing great about it. If Pope Gregory XIII, who introduced the calendar, had opted 30 days for January, it would not be called the blue moon.
Viewing the supermoon
The eclipse at moonrise is something that is enjoyable. All you need to do is just occupy a place which provides the best visibility of moonrise. 'What colour will we see?' is a question that many will have. The colour of the moon may be orange, brown or copper. You do not need any special equipment to see the supermoon. In case you have telescope or binoculars, search for a faint star cluster very close to the moon.
The eclipse will start at 4.20 pm. So, you will be watching the moon at the midpoint of the eclipse when it is in the umbra - the darker shadow region. Gradually, you can see the edge getting brighter. By 9.40 pm, the full moon is clearly visible. The star cluster that you had seen earlier is not visible any more.
Try and take a picture of the sequence from moonrise onwards. You will see that the size of the moon is the same in all the images. By doing so, you will disprove the myth that moon 'looks' big at the horizon and then gets smaller. Also, remember to take pictures of the lunar eclipse on July 28. That is the only way you can verify the meanings of apogee, perigee, supermoon or micromoon!
(The author is visiting scientist, Jawaharlal Nehru Planetarium, Bengaluru)