Just a Minute- What's all this about Time?

An article on by Brian Sheen

As astronomers we get involved in three basic types of time. These are;-

Apparent Solar Time.

This is the time indicated by the Sun as it passes across the sky. The Sun rises in the east, passes south at midday and then sets in the west. However that is an over simplification and due to the fact that the Sun tracks along the ecliptic and the Earth's orbit is elliptical it may be due south either 15 minutes early or 15 minutes late. Then of course as we are some 4 degrees west of Greenwich the Sun takes a further 16 minutes to transit our meridian. In addition during the summer we advance our clocks by one hour! Apparent Solar Time is the time kept by a Sun Dial and is the time we would live by if we did not have clocks to straight jacket us. This brings us neatly on to …..

Greenwich Mean Time.

Clearly the Sun is nothing like constant enough for we moderns to run our lives by and so a more uniform system was introduced. This relies on a hypothetical Sun tracking over the equator at a regular rate throughout the year. It is based on measurements taken at Greenwich and Greenwich Mean Time is used throughout the UK. GMT was adopted as the basis for the world's time during a Conference in 1884 held in Washington. The British Team was led by John Couch Adams the famous Cornish Astronomer. The Observatory at Greenwich is the home of the Prime Meridian of the World. The actual measurements were not taken of the Sun, it is difficult to find the exact centre of such a large disc and so the transits of stars were recorded with great accuracy to give a basis for Sidereal Time.

Sidereal Time.

As you all know the Earth rotates on its axis not 365 times in a year but actually gets in an extra rotation during the orbit giving 366 rotations per year. This means that stars transit our meridian 4 minutes earlier every day. The start point for this is the Vernal Equinox and this point transits at midday on the 21st March. By the 21st April it transits 4 X 30 minutes earlier ie at 10:00 hours. The connection with our sort of astronomy is that the Vernal Equinox has a Right Ascension of 0 hours 0 minutes.

The famous Double Cluster in Perseus has a RA of 2 hours (note minutes disregarded for this purpose). This means it transits at 1400 on the 21st March - clear so far?

What should be clear is that the transit times illustrated are during the day and therefore cannot be seen. We therefore have to refer to the autumnal equinox the 21st September, 6 months later, and this means that the Vernal Equinox, Right Ascension 0 hours 0 minutes is due south at midnight and that the Double Cluster in Perseus transits at 0200. If you know our local sidereal time is you can find out which stars are due south at that time!

As we have stated before Sidereal Time keeps changing relative to GMT and so the best thing is to check it out using the following web site http://tycho.usno.navy.mil/what.html and go to compute Local Sidereal Time. If the local sidereal time is 16 hours it means that a star with an RA of 16 hours will be due south. Before radio became widespread local people in the villages kept a track of stars transits to regulate or check the accuracy of their clocks.

An extension of this process can be used to determine due south or if this known the exact local time can be established. Use a planetarium programme on a PC to check all this out.