I am trying to answer the following question which I slightly understand but need a clearer explanation:

What are the times of year to observe an object at X hours of RA at visual wavelength?

I am trying to use the march equinox as a starting point for 0 RA and for a 12 hour RA object, it will begin to rise from the east as the sun sets west of course, around 6 months from the march equinox???

I appreciate an explanation and if you can link a map or a helpful visual as well. Thanks!

  • $\begingroup$ It depends on the declension and your latitude: For example Polaris is visible all year for Northern hemisphere observers (and never visible for southern) Other stars are visible for different amounts of time: Sirius is visible between mid August and Mid April where I am, (It is different at other latitudes). $\endgroup$
    – James K
    Aug 30, 2020 at 14:00
  • $\begingroup$ Say for example on 37 N latitude and an object at RA 6 hours. $\endgroup$
    – echo12
    Aug 30, 2020 at 14:30
  • $\begingroup$ and its declension? $\endgroup$
    – James K
    Aug 30, 2020 at 14:42
  • $\begingroup$ Observer somewhere in California 37N 119 W. And object 6 hrs RA $\endgroup$
    – echo12
    Aug 30, 2020 at 14:47
  • 1
    $\begingroup$ I think James meant declination. $\endgroup$
    – Mike G
    Aug 30, 2020 at 16:50

1 Answer 1


For an object at about RA 6h and close to the celestial equator, viewed from 37 degrees N, it becomes visible in the early morning on about July 20th, and it is is visible at some point during the night until about May 20th, when it sets shortly after the sun. Between May 20th and July 20th it is not visible. (Determined Stellarium)

An example of such an object is Betelgeuse.

An object that is at 6h of right ascension will culminate at midnight (ie reach its highest point at midnight about on the winter solstice on Dec 21st. The sun has a right ascension of 6 hrs on the summer solstice on June 21st and a RA of 18 hours on the winter solstice. So the best time to observe the object would be on the winter solstice.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .