Is it ever visible from here?

How can I tell whether a star or constellation is ever visible from my location?

It's simple if you know your latitude and the star's declination! For example, let's say your latitude is 40 degrees north, and you want to know if the brilliant star Canopus (alpha-Carinae), declination -53 degrees south, is ever visible from your location. Follow these steps:

  1. Subtract your latitude from 90 degrees. This result is your co-latitude.
    Example: 90° - 40° = 50°
    The co-latitude of 40° north is 50°.
  2. To your co-latitude, add the declination of your star/constellation. (Remember to treat southern declinations as negative numbers!)
    Example: 50° - 53° = -3°
  3. If your co-latitude plus the declination of the star is greater than zero, then it will rise above your horizon at least sometime during the year.

Another way to think of this is if the sum of your latitude and the absolute value of a southerly declination is less than 90°, then it's visible.

  • Visible if: [(your latitude) + | southerly declination | ] < 90°

The example calculation shows that Canopus would only peak above the horizon from a location south of 37 degrees north latitude; to see it clearly one would have to travel to the southern United States, around 30 degrees N.


Test your understanding:

If the celestial equator has an altitude of 55 degrees above the south horizon at a location 35 degrees north terrestrial latitude, then what is the most southerly declination visible from this location?