What might be the reason that the moon looks comparatively larger and brighter than the stars and planets that are seen in the sky at night?
The moon is the celestial body that is nearest to the earth, and it is the only natural satellite of the earth. Its gravity is less than that of the earth. There is no water and no atmosphere in it. There are mountains and plains on the moon as there are on the earth, but there are no living beings.
Distance, and nothing else. The moon is not big; its diameter is 3,476 kilometres, which is barely a quarter of the earth and far smaller than any planet. It is not bright either, in the sense of making light, because like every planet it makes none at all and only throws back the sunlight that lands on it. What it has is nearness. At an average of 3,84,400 kilometres it is the closest object in the sky by an enormous margin, and everything else you can see at night is millions or billions of kilometres away. This is the same explanation that was given for the sun two topics ago: the sun is an ordinary star and looks enormous because it is near. So the two brightest things in your sky, one by day and one by night, are both ordinary objects that happen to be close, and that single idea has now explained both.
Because the moon travels in an ellipse when it orbits the earth, it does not always stay at an equal distance from the earth. The average distance from the earth to the moon is 3,84,400 km. Its diameter is 3,476 km. The area of its surface is 3.79 × 10⁷ km². And it takes about 27 days to complete one full orbit of the earth.
The phases of the moon
The lit part of the moon does not always look the same. After the new moon its shape begins to be seen like a sickle. The lit part goes on increasing and it is seen fully round. After that the lit part goes on decreasing, reaches the sickle shape again, and at the end becomes invisible. This sequence keeps repeating without a break. The lit part of the moon, and the difference that is seen in its shape, is called the phase of the moon.
| The reasons the phases are seen |
|---|
| The moon orbiting the earth in its own orbit. |
| The moon itself being a non-luminous object. |
| Every day the moon reflecting light towards the earth from a different area of its surface. |
This is the sentence that makes the whole topic simple, and the book never quite says it. Sunlight comes from one direction, and the moon is a ball, so one half of it faces the sun and is lit while the other half faces away and is dark. That is true every single night, without exception, including at the new moon when you can see nothing at all. Nothing about the moon changes as the month goes by. What changes is where you are standing to look at it. As the moon travels round the earth you see the lit half from different angles: sometimes almost from behind, so you see hardly any of it and call it a new moon; sometimes from the side, so you see half of the lit half and call it a quarter; and sometimes from in front, so you see the whole lit face and call it a full moon. A phase is not a change in the moon. A phase is a change in your view of an object that never changes at all. Once that is clear, all four of the names later in this topic explain themselves.
Two days after the new moon a small part of the moon can be seen in the sky, and this is called waxing crescent. A few days before the full moon arrives, only a very small part of the moon looks dark, and this is called waxing gibbous. In the same way, after the full moon usually only a small part of the moon stays dark, and this is called waning gibbous. As the dark part goes on increasing, a few days before the new moon arrives only a little part is seen lit, and this is called waning crescent.
Those four English names look like four things to memorise and they are not, because each is made of two words and each word means one simple thing. Waxing means growing, so the lit part is getting bigger and the full moon is coming; waning means shrinking, so the lit part is getting smaller and the new moon is coming. Crescent means less than half is lit, the thin sickle shape; gibbous means more than half is lit, the fat lopsided shape. Put one word from each pair together and you have all four names, and you can also work out which one you are looking at in the sky tonight. Waxing crescent: growing, less than half. Waxing gibbous: growing, more than half. Waning gibbous: shrinking, more than half. Waning crescent: shrinking, less than half. In Nepali the same idea is carried by the two paksha: shukla paksha is the waxing half of the month and krishna paksha is the waning half.
Four words for four positions
| Term | What it means |
|---|---|
| Full moon (Purnima) | The state in which the shape of the moon is seen fully lit and round from the earth. On this day the EARTH falls between the sun and the moon. In figure 11.27 the 30th is the full moon. |
| New moon (Aunsi) | The state in which the moon is not seen from the earth. On this day the MOON falls between the sun and the earth. In figure 11.27 the 15th is the new moon. |
| Bright half (Shukla paksha) | The period from the new moon to the full moon. In it the lit part of the moon goes on increasing gradually, and it lasts about 15 days. In figure 11.27 the 16th to the 30th is the bright half. |
| Dark half (Krishna paksha) | The period from the full moon to the new moon. In it the lit part of the moon goes on decreasing gradually, and it lasts about 15 days. In figure 11.27 the 1st to the 15th is the dark half. |
Read the first two rows again and notice what they are describing. At the full moon the earth is directly between the sun and the moon. At the new moon the moon is directly between the sun and the earth. So twice every month the three bodies come into a line, and that is exactly the arrangement that produces an eclipse: at the full moon the earth could throw its shadow on the moon, and at the new moon the moon could throw its shadow on the earth. Which raises an obvious question. If they line up twice a month, why is there not an eclipse twice a month? Everybody knows there is not; eclipses are rare enough to be events. Hold that question. The answer is in the next topic, and it turns on one small detail about the moon path that nothing so far has mentioned.
Two kinds of month
The period the moon takes to orbit the earth once is called the sidereal month, and it is about 27 and one third days. In the figure, the time the moon takes starting from point 1 to make one round of the earth, or to arrive at the place where point 2 is, is the sidereal month.
The period from one new moon to another new moon, or from one full moon to another full moon, is called the synodic month, that is, the lunar month, and it is about 29.5 days. Every new moon starts a new lunar month. When the moon has made one round of the earth, the earth has also moved some way ahead in its own orbit, from point 2 to point 3 in the figure, and reaching the next new moon takes about 2 days and 5 hours more than 27 and one third days. That is why the synodic month is longer than the sidereal month.
The difference between those two months confuses almost everybody the first time, so here is another way to see it. Imagine running round a circular track while the finishing post is slowly being carried forward. You complete one full lap and arrive back at the spot where you started, and that is the sidereal month: the moon has genuinely gone all the way round the earth. But the post has moved on while you ran, so to cross the line you have to keep running a little further, and that extra bit is the two days and five hours. The finishing post here is the direction of the sun, and it moves because the earth is carrying the moon along with it as it travels round the sun. So the sidereal month asks where the moon is against the distant stars, and the synodic month asks where the moon is against the sun. The second one is longer, and it is the one that matters to a calendar, because what people see in the sky is the phase, and the phase depends on the sun.
When the 360 degree angular measure of the circle the earth makes in orbiting the sun is divided into twelve equal parts, the time the earth takes to cross the resulting 30 degree angle is called one solar month; however many days the earth takes to cross 30 degrees, that month is of that many days, and generally a solar month is of 30 to 31 days, while February is of 28 days and in a leap year of 29. The solar year is of 365.25 days, while the lunar year is of 354.37 days. We take one solar year as 365 days, which is 0.25 days less than the actual solar year; the time left out every year is added together, and after four years one day is added to make a year of 366 days, and that 366 day year is called a leap year.
Adhik maas and kshay maas
In daily practice the solar month is used, but because festivals and religious activities take place according to the lunar month, both months have been carried alongside each other. The lunar month of every month starts from the new moon, and whichever solar month a lunar month starts in, the lunar month is named after that solar month. But the days on which these two months begin may not be the same. Sometimes, if the solar month has many days, two new moons can fall in a single month, and in that case both lunar months have to be given the name of the same solar month.
For example, in B.S. 2077 in the month of Asoj, the 1st was a new moon day, so the lunar month of Asoj started from that day. On the 30th of the same month another new moon fell, so the other lunar month also had to be named Asoj, and the Asoj lunar month came to be repeated. The repeated lunar month in such a state is called adhik maas, malmaas or purushottam maas. Generally one adhik maas falls every three years.
In the process of matching the solar month with the lunar month, when the moon is in the state of being farthest from the earth the lunar month becomes somewhat longer, and when the earth is nearest to the sun the solar month becomes shorter. When that happens, not even one new moon may fall inside the short solar month, so in the process of matching the two, a lunar month gets broken in some solar month. This breaking of a lunar month is called kshay maas. In a year that has a kshay maas, two malmaas can fall, and kshay maas particularly falls between Kartik and Magh.
The whole of adhik maas comes down to two numbers that were given a page earlier and never put side by side. A solar year is 365.25 days. A lunar year of twelve lunar months is 354.37 days. So a lunar year is about eleven days shorter than a solar year, and that gap does not stay small: after three years it has grown to about a month. If nothing were done, the festivals would drift steadily backwards through the seasons, and within a few decades Dashain would be arriving in the middle of the monsoon and then in the winter. Nobody wants a harvest festival that has wandered away from the harvest. So an extra lunar month, adhik maas, is inserted about every three years, which pushes everything back into place and keeps each festival in its proper season. Kshay maas is the same problem seen from the other side, when the arithmetic runs the other way and a lunar month has to be dropped. This is not superstition or arbitrary tradition. It is careful astronomy, worked out centuries ago, solving a real arithmetical problem, and the whole of it exists because two clocks in the sky do not divide into one another evenly.
Everybody in your class has seen the moon hundreds of times, and every one of those times they were looking at the same face. The dark patches make a familiar pattern, and it never turns round. That is because the moon takes about the same time to spin once on its own axis as it takes to travel once round the earth, so as it goes round it turns to match, and one side is kept permanently pointing at us. Nobody on earth saw the far side of the moon at any point in human history until a spacecraft went round and photographed it in 1959. Two things are worth being careful about. That far side is not a dark side; it gets just as much sunlight as this side does, and when we see a full moon the far side is in complete darkness. And this is not a coincidence: the pull of the earth on a nearby body slowly brings its spin into step with its orbit over a very long time, and many moons in the solar system have ended up the same way.
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