Depending on our location on the earth, we can see only certain stars and constellations. At the North Pole, the pole star appears directly overhead. But this star is not visible to people living in the Southern Hemisphere. Even in India, which is a vast country, the night sky over Cochin looks different from that over Guwahati. Not only does the night sky look different from different locations on the earth, its appearance at a place changes in the course of a night and also in the course of the year.
When we talk of the sky at a place, what exactly do we mean? Imagine standing in a large, flat field. The sky around you will appear as a hemisphere, or a dome, with you at its center (Figure 10.19). All celestial bodies such as the sun, the moon and the stars will appear to be traveling across the surface of this hemisphere. For example, every day, the sun rises over the eastern horizon, appears to travel westwards across the surface of this hemisphere and sets over the western horizon. The stars also appear to move across the surface of this imaginary hemisphere. Depending upon our location on the earth, we see different sets of stars in this hemisphere.
Change in positions with the time of the night The positions of the stars and constellations appear to change with the time of the night. The position of a constellation or a star at 10 p.m. is different from that at 8 p.m., and so on. This is because we look at the stars from the earth, which rotates through 360 degrees in 24 hours, i.e., 15 degrees per hour, or 1 degree in 4 minutes. The stars therefore appear to rotate at this rate in the opposite direction. In other words, the position of a star appears to change by 1 degree every 4 minute. For example, if a star is at the horizon at 10:00 p.m., it will be about 1 degree above the horizon at 10:04 p.m.
Change in positions from night to night. The revolution of the earth round the sun also affects the appearance of the night sky. What you see today is different from what you will see tomorrow at the same time of the night. If a particular star is on the eastern horizon today at 10:00 p.m., tomorrow it will be slightly above the eastern horizon at 10:00 p.m. The difference may not be noticeable if you compare the positions on consecutive nights, but if you compare it with the position at the same time after a month, the difference will be obvious. But the appearance of the sky at the same time after a year should be the same as it is today. This is because the earth completes its orbit round the sun in 365 days. Simple arithmetic will tell you that 360 degrees in 365 days means a change of about 1 degree every day. Thus, a star that appears on the eastern horizon on one day at 11:00 p.m. will be found about 1 degree above the eastern horizon at the same time the next day. In other words, it would be at the horizon at 10:56 p.m., since we have seen that the position of a star appears to change by 1 degree in 4 minutes. This means that every day, a star rises four minutes earlier than the previous day. Also, a star that appears at the eastern horizon at a particular time on a particular day appears at the western horizon at the same time six months hence, having traveled 180 degrees in 180 days.
Appearance of the Night Sky
On a particular night, a star rises on the eastern horizon at 9:30 pm. At what time will it rise after 15 days?
Solution A star rises 4 minutes earlier than the previous day. So, after 15 days, it will rise 60 minutes earlier, i.e., at about 8:30 p.m.
Star maps It is possible to create the image of the sky as it would look on a particular day and time at a given location. Such an image is called a star map. A star map helps us locate the various constellations and stars. Figure 10.20 is a simplified star map of the Northern Hemisphere for the month of October, with a few constellations marked out for you.
The Changing Positions of the Sun in the Sky
How do the earth's rotation and revolution affect the appearance of the sun in the sky? Because of the rotation of the earth, the sun seems to rise over the eastern horizon, move across the sky in a westerly direction throughout the day, before setting over the western horizon. However, the sun does not rise from the same position every day. This is because of the earth's revolution around the sun. In the Northern Hemisphere, after 21 June, the position from which the sun rises keeps
on shifting towards the south. Its path across the sky also changes, as shown in Figure 10.21. Then at one moment on 22 December, the sun seems to reach its southernmost position in the sky. After this, it begins moving northwards over the next six months, reaching its northernmost position on 21 June. Before changing directions on these two days, the sun seems to become still for a moment. That moment or the sun's position then is called solstice (meaning 'the sun is still'). The northward and southward journeys of the sun are called uttarayan and dakshinayan respectively in many Indian languages.
Because of the apparent north-south motion of the sun, the sun does not attain an exactly overhead position at noon on most days. It does so only twice a year, but only at the equator. These two days are 21 March and 23 September. The moments on these days at which the sun is directly overhead are called equinoxes. On these days, the path of the sun is midway between those on the days of the solstices. The days and the nights are equal in length on the day of an equinox (equinox means equal night). Also, only on the day of an equinox does the sun rise exactly from true east and that too only at the equator. On all other days, the positions of sunrise and sunset cannot be taken as the true positions of east and west respectively.
Finding directions How do you find directions (north, south, east, west) if you do not have a compass? At night, the pole star will indicate the north, and then the other directions can be found easily. To find the directions during the day, fix a rod, about one metre in length, vertically in an open space. Mark the position of the top of the rod's shadow on the ground (Figure 10.22). Repeat this at an interval of, say, 10 minutes from 11 a.m. to 1 p.m. Join these points to get a curve. Find the point B on this curve that is closest to the bottom, A, of the rod. Join AB, which is the length of the minimum shadow. The direction from A to B gives the direction of the true north-south.