When Can We See Them?

How long does it take the Earth to spin?

No doubt you will have answered 24 hours. That is actually wrong. It takes the Earth 23 hours and 56 minutes to spin on its axis. If you could see the stars all the time you would notice that they move a little bit further than 360 degrees in 24 hours. About 1 degree more.

24 hours is just the average time between mid-day (when the Sun is at its highest) and the following mid-day for all the days throughout the year.

The reason there is a difference is because, relative to the Earth, the Sun moves a little bit quicker than the stars.

• Every day it moves about an extra degree in the sky.
• Every 30 days or so it moves through one of the zodiacal constellations. What constellation was the Sun in when you were born?
• Every year it makes a complete revolution in the sky.

So, not only do all the stars appear to rotate one around Polaris every 24 hours (almost) if we looked at the stars at the same time every night we would notice that they rotate about 1 degree extra every evening, doing 1 complete extra rotation every year.

Why is Pisces best viewed in August / September?

Imagine we looked at the position of the stars at midnight on the same day every month throughout the year. Because the Earth takes a little less than 24 hours to rotate the every month the stars would be further anti-clockwise by about 30 degrees. After a year they would be back where they started from. I am starting this celestial clock on March 21st because on that day the celestial meridian is aligned north south. I am using the celestial meridian as the hand of my clock.

The Sun is in Pisces around February / March. This means that Pisces is in our sky during the day and so is impossible to see. In August September the Sun is on the other side of the celestial sphere well away from Pisces. Pisces is in the sky at night so is visible at this time.

Some constellations are well known as winter constellations and others as summer. Constellations which contain stars all circumpolar stars, i.e. they have relatively big declinations, are visible throughout the year, e.g. Ursa Major and Casseopeia.

How does your Latitude affect what you can see?

The further North your latitude is the more constellations that you can see will be circumpolar. If you imagine being at the North Pole, with Polaris at your zenith, then all the constellations you can see would be circumpolar, i.e. they would all rotate around Polaris directly above your head. If you lived on the equator then no constellations would be circumpolar.