18/06/2021
---------------------Cycles in Sky---------------------
-Pritam Bala
If you go out in the night sky, you’ll see thousands of stars. They rise in one direction and hours later, sets in another. The stars rise up , they stay for a few hours , the Polaris hangs on the north and so on. If you go to the next day , it’s pretty much the same . But if you wait for a day , a week or a month , you will see slight and subtle change.
If you notice a star rising at a certain point of year from the horizon , you’ll see it rise more above the sky after few months. If you wait a month, this effect is even more pronounced; all new constellations will be visible in the sky after sunset. This is because the Earth is going around the Sun, literally changing our viewpoint on the sky. The Earth takes a year to orbit the Sun once. Every day, it moves a little bit along its orbit. And as it does, from Earth's perspective, distant stars appear to move their positions relative to the sun. So, one day we might see a star very near the Sun, but the next day the angle is a bit bigger. At some point, about six months after we first saw it, the star is directly opposite the Sun in the sky. Then the angle starts to shrink again as the star approaches the Sun from the opposite side, until, after a full year, the cycle repeats. What this means to you, the naked eye observer, is that the stars appear to rise and set at different times over the course of the year.
Stars in the east rise about 4 MINUTES earlier every night, and stars in the west set 4 MINUTES earlier. A constellation that was entirely below the eastern horizon at sunset one month might be completely visible after sunset the next month.
Another way to think about it is that the stars appear to be fixed, and as the Earth circles the Sun, the Sun moves through the stars over the course of the year, making a complete circle around the sky once per year. The path it takes is a reflection of the Earth's path around the Sun, a line in the sky. We call that line the “Ecliptic”. That means the Sun passes through the same constellations in the sky every year. We give those constellations a special name: “The Zodiac”. Every year, during a given month, the Sun will appear to be in a certain zodiacal constellation, from Sagittarius through Scorpius, Libra, Virgo, Leo, Cancer, and the rest. Eventually, over a year, the Sun returns to ‘Sagittarius’, and the cycle starts again. But even though we talk about this process in terms of the sun's movement, it's really the path traveled by the Earth that creates this effect, as our perspective moves with it. And of course, the planets move in the sky as well. Mercury, Venus they orbit the Sun, too, and they do so in approximately the same plane the Earth does. If you could see the solar system from the side, it would look flat! So to us, on Earth, the planets go around the sky over the course of a year, and they also appear to change their positions relative to the Sun and the stars. The inner planets, Mercury and Venus, move so rapidly you can see their motion after a single night. The outer planets are more slowly, but wait long enough and they too will be seen to move, sliding through the constellations.
They all have their own orbit to complete , and we , as a observer analyze the path to determine it’s location and other geometry. That’s it for today , stay tuned for next article.
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