The Equation of Time
This is a table of the amount by which the Sun is fast or slow each day compared with the average time that we use for our everyday life – and which is why, for example, Greenwich Mean Time is the mean, or average, of the variations in solar time.
The reason this is necessary is that the Earth's orbit around the Sun is not a perfect circle but is an ellipse. When the Earth is comparatively close to the Sun (as it is in January), it moves faster in its orbit than when it is farther from the Sun, as in June.
As a result, when the Earth is moving faster than average in its orbit, the Earth doesn't rotate on its axis quite enough during one day to bring the Sun due south at 12 noon by our watches, and it is slow.
Here is a table giving the approximate dates when the Sun is fast (–) or slow (+) compared with your watch by a whole number of minutes:
How does this affect you? If you are trying to find your north-south line by day, make the correction to find out when the Sun is exactly on your meridian. For example, on 2 March the Sun is due south at 12.10 pm. Or if you have a well-made sundial, add or subtract the figures shown to get the local civil time. If the sundial reads 3.45 on 17 November, the time by your watch should be 3.30 pm.
Actually, there are other factors affecting sundial time, of which the main one is shoddy manufacture, which is the problem with most of the sundials you buy in garden centres these days... but don't get me started, that's another story. With a well-made sundial, the other correction you must make is for your distance from the meridian of the time zone you are in. Even in a small country such as the UK this can be considerable. Remember that every 15º of longitude makes an hour of difference in the solar time, or 4 minutes per degree. So in Cornwall, west Wales, western Scotland or Northern Ireland you are about 5º west of the Greenwich meridian and the solar time is 20 minutes later than GMT. On 5 February in Belfast, therefore, a sundial will not tell you that it is noon until 12.35 on your watch, and in early August it will be 1.28 pm when the sundial reads noon, because of British Summer Time.
Once you have made these corrections, a properly made sundial should give you the time just as accurately as most digital watches, which can easily drift off by half a minute or more unless they are regularly checked.
For a full listing of the Equation of Time for every day in the year go to
http://freepages.pavilion.net/users/aghelyar/sundat.htm (separate site).
|What's in the sky tonight?
Below is a listing of the positions of the planets for this year. They are calculated for an observer at the centre of the Earth, so they apply equally to everybody -- the difference between locations does not matter at this level of accuracy. But if you just want to know what the sky will look like tonight, where you are, try the website www.heavens-above.com. Once you have registered (free) with your own location you can get a whole-sky map (under Astronomy) and get your own predictions for bright satellites such as the International Space Station and the Iridium satellites which flare in brightness.
Positions calculated using Chris Marriott's SkyMap
positions every 5 days during
western and eastern elongations. El = elongation from Sun in degrees.
|Venus – positions every
10 days. El = elongation from Sun in degrees.
|Mars – positions every
|Jupiter – positions
every 10 days
|Saturn – positions
every 10 days