## Latitude from Circumpolar Stars

Circumpolar stars are stars that stay above your horizon and they can be used to find your latitude (shown in Figure 16.3).

DUBHE Zenith 82 42'

DUBHE Zenith 82 42'

Polaris is behind the clouds all night. (And Dubhe is not?)

Altitudes related to 55 N.

Add altitudes together and divide by two for an answer which is a shade over 55 but there are a couple of catches. Sights must be 12 hours apart. Possible in the tropics but not in temperate latitudes during summer. Secondly, during that time you mill have changed position so any answer will be approximate; even more so if you take a nadir sight one night and the zenith sight the next.

Polaris is behind the clouds all night. (And Dubhe is not?)

Altitudes related to 55 N.

Add altitudes together and divide by two for an answer which is a shade over 55 but there are a couple of catches. Sights must be 12 hours apart. Possible in the tropics but not in temperate latitudes during summer. Secondly, during that time you mill have changed position so any answer will be approximate; even more so if you take a nadir sight one night and the zenith sight the next.

16.3 Latitude by Circumpolar Stars

1. First identify your circumpolar star.

2. Measure its altitude when it is at its nadir. This is its lowest point in the sky and it is directly below the Pole.

3. Wait until it is at its zenith when it is directly above the Pole and measure its altitude again.

The average of your two readings, duly corrected, is the altitude of the Pole and so your latitude. It does not matter if you do not know which star(s) you are using or even their declination. The drawback is that the sights are 12 hours apart so you are confined to circumpolar stars that are on the zenith (or nadir) at dusk and then at the nadir (or zenith) around dawn.

16.4 Zenith Stars Zenith Stars

Since a star is at its zenith somewhere over the earth all day, every day this means that all stars circle the earth at their zenith (Figure 16.4). The angle that a star makes with the centre of the earth and the equator is called its declination. In earthly terms there is no difference between a star's declination and latitude. So when a star of a known declination is directly overhead then you know your latitude.

Imagine that you are sailing down your latitude to your chosen destination. In the northern hemisphere Polaris provides an easy latitude check but what if it is obscured, or if you are in the southern hemisphere?

Once you are on the latitude of your destination become familiar with stars passing through your zenith. It is not necessary to know them by name, only that they

Markab has a declination of 15 N. -

You estimate that when Markab is on its zenith that the angle between it and your zenith is about

It is south of you so your latitude is about 35 N

16.5 Latitude by ex-Meridian Stars are your zenith stars and they confirm you are on course even if you do not know their declination.

If a star whose declination you know is almost, but not exactly, on your zenith, then you can try estimating the angle between it and your zenith (Figure 16.5). This will not be easy and any answer is bound to be rough and ready but it may help to confirm your latitude. If you reckon the star is 10° north of you then your latitude is 10° south of its declination.