To use your wristwatch or smart phone to tell the time at night is to ignore the celestial clock in the northern night sky (for those in the northern hemisphere). All self-respecting astronomers will of course use a couple of pieces of cardboard marked with a representation of a few star patterns around Polaris (the North Star) to make their northern hemisphere clock. They then read the time by comparing with those patterns in the sky. Here is a guide so that you can make your own Star Clock. With practice you will achieve great precision in reading the time this way - perhaps with an error of no more than 30 or 40 minutes! (The clock itself is quite accurate. The limitation is in judging the match between clock and sky by eye.)

The inner dial should turn nicely against the outer dial (loosen the fastener slghtly if not). To read the time:

• Face north and look at the night sky (it helps if the weather is kind.)

• Hold the clock vertically in front of you, with the current date at the top. For example, in mid April hold the clock with the centre of the Apr segment at the top.

• Turn the inner dial until Cassiopeia and The Plough align with the way you see them in the sky.

• Read the time on the outer dial that is lined up next to the Time arrow on the inner dial. You now know the time and have communed with the stars!

Note that the clock reads UTC time, which for us is just a fancy way of saying Greenwich Mean Time. In Britain, this is handy so that normally we can read the time directly, unless British Summer Time is in effect, when we must add one hour to the time read from the Star Clock.

If you want to practice using the clock, you can use it in reverse and check against the time on your wristwatch. Hold the star clock as normal, but now turn the inner dial until the current time is aligned with the time arrow. Now looking at the sky you should see the star patterns in roughly the same alignment as on your star clock. (For British Summer time, subtract one hour from your wristwatch time to use with the star clock.)

In the Northern hemisphere, we are lucky that Polaris happens to be a bright star that is very close to the North Celestial Pole in the sky. This is the point about which the stars seem to turn as the Earth rotates. Another way to think about it is that when you look at Polaris you are looking parallel to the Earth's axis. As the Earth turns, the star patterns appear to turn in the opposite direction about Polaris. (The stars are so far away that the size of the Earth and even the size of Earth's orbit around the Sun are negligible, so we can think of a pure rotation.) Polaris is the centre of our Star Clock! For those who have seen a Planisphere, the Star Clock is a bit like a planisphere with very simple star markings and being used in reverse - reading the time from the observed alignment of the stars on a given date.

Why do we need to adjust for the current date, in the way we set the date at the very top of our clock when we hold it? This is because the time we use - 24 hours per day - is not quite the same as sidereal time, which is the time measured by Earth's rotation against the background of stars. The Earth actually does one full rotation in about 23 hours, 56 minutes. So the star patterns advance by about 4 minutes from one night to the next.

Why do we have 24 hour days for our time measurement then? Because 24 hours is the time (averaged over a year) that it takes for the Sun to advance from one Noon (highest point in the sky) to the next. It is a little longer than the sidereal time, because the Earth has advanced a little bit in its orbit in one day, so must rotate a little more than a full turn in order that the Sun be back in the same direction in the sky.

Happy star gazing!

Mike