Time
Two Minute Astronomy, 5
We’ve laid down the foundation to understanding the position and movement of celestial objects up until now. Let’s explore further about how exactly the rotation of Earth and our frame of time play a role in astronomy.
So the first tidbit we should know is that, the time taken by our planet for one rotation on its axis and the time taken by the Sun to complete one full loop in our sky are not equal. Why? The Earth is also moving along its orbit (revolving around the Sun) at a very high speed, and hence the Earth needs to catch up by having to rotate a bit more than one full rotation, so that the Sun is found near the same place as it was yesterday. In fact, the Earth’s rotation time (one sidereal day, 23 hours, 56 minutes and 4.1 seconds) is nearly four minutes less than the time our Sun takes to finish one ‘revolution’ around our us (one solar day, around 24 hours). Since in most day-to-day contexts we only use solar time, this mismatch does not affect our usual functioning.
But wait. While this might not make a huge difference in how we use the Sun to measure our days, there’s another aspect we need to consider as astronomers. Say we are looking at a star when your clock reads 8 PM tonight. If the Earth takes only 23 hours and 56 seconds to complete one rotation, then tomorrow the same star will reach the same position as it was tonight at 7:56 PM itself! The 4 minute difference does matter now because with each day, any given star on the celestial sphere will appear to rise and set (that is, cross the horizon) earlier and earlier throughout the year. Because of this, the night sky on a winter night at 8 PM is not the same as the night sky on a summer night at 8 PM. A different section of the celestial sphere is visible each night, with some constellations visible only at some times of the year (of course they are always present on the celestial sphere, but sometimes they are either up during daytime or are visible only in the very early hours of the morning).
It’s almost like having old friends who drop by once a year and stay for a while before parting ways — bittersweet, yet a celebration of sorts. It is also how our ancestors kept time for millennia: in ancient Egypt, the annual flooding of the Nile Valley coincided with the star Sirius rising just before dawn, in late summer. Since the flooding left the plains rich in silt and thus fertile, this celestial occurrence was a very important event in their calendar.
So we’ve had a basic introduction about how the sky changes over time. Next week, we’ll explore how the place you live in determines what parts of the celestial sphere you see. Until then, stay tuned!
