## Circumpolar Stars

- Describe what is meant by the term 'circumpolar stars' and explain the connection between the apparent motion of stars and the Earth's rotation
- Demonstrate an understanding that a star will be circumpolar from a given latitude provided declination > 90 - latitude

Circumpolar stars are stars that can be seen from one location throughout the year. Other stars are usually only seen for a month or two.

Circumpolar stars are caused by the axis of the Earth's tilt; the same reason why we have the seasons.

They are stars that never seem to set below the horizon, so the nearer you are to a pole, the more stars you’ll see that are the same throughout the year, and the less seasonal stars. When you’re nearer the Equator you’ll see less circumpolar stars but more seasonal stars.

Stars appear to revolve around Polaris in the Northern Hemisphere. As the Earth rotates stars appear to revolve around that star because it is above the Earth’s northern axis.

In the exam you may be asked to determine:

• Which stars are circumpolar and which are not.
• What latitude you would have to be at in order to see a certain star
• The smallest or largest Declination a star would be at from a given latitude

The formula for working out if a star is circumpolar or not is:
D >= 90° - L

D = Declination
>= = Greater than or equal to
L = Latitude

From Britain, the further north you travel the more circumpolar stars you see, and the further south you travel the less you see, though there are more stars in the sky. The reverse is true if you live in the Southern Hemisphere.

From London (51°N), if a star is greater than 39° then it will never set. However if it is less than 39° it will never be seen from London.

##### Question 1

Deneb = Dec 45 degrees
Arcturus = Dec 19 degrees
Alpha Centuri = Dec -61 degrees
If you were in London would Deneb be circumpolar?

D greater than or equal to 90 - L
90 - L (51) = 39
45 is greater than 39 so Deneb is circumpolar
Arcturus (19N) is less than 39 so is not circumpolar but seasonal
Alpha Centuri has a declination -61 is less than 39 and so would not be viewable from London.

##### Question 2

How far south would you have to be to see Alpha Centuri?
Tip - treat negative numbers as positive and then convert them afterwards.

So to see it add 90 to -61 which makes 29 so you would have to travel to latitude 29N to see this star.

##### Question 3

How far south would you have to be to see Alpha Centuri as a circumpolar star?

To see it as a circumpolar star = 90 - 61 = 29 so you would have to be at least 29° south to see it as a southern circumpolar star.

###### Summary

D >= 90° - L

 D Declination >= Greater than/equal to L Latitude
###### Bonus Question

State whether or not each of these stars would be:
Visible from New York (41°N) or Sydney (34°S) and circumpolar from either city or seasonal.
Also state the minimum latitude (north and south) they would have to be seen from to be circumpolar.
Key: New York = NY, Sydney = SY.

Star Dec. V C ML
NY SY NY SY NY SY
Capella 46N
Betelgeux 7N
Vega 39N
Antares 26S
Alpha Crux 63S

Visible?
Circumpolar?
Min Latitude?