# Is right ascension only measured once a year?

• B
So I'm learning some basics about astronomy...and this right ascension value is defined as being measured eastward along the celestial equator from the vernal equinox....now as I understand it, the vernal equinox only occurs once a year (the first day of spring, in the northern hemisphere), so is any object's right ascension value only measured on one day of the year, and then kept constant until the vernal equinox comes around again in a year?

Chronos
Gold Member
You gotta set the zero point somewhere in the sky and astornomers chose the vernal equinox as the zero point for the measurement known as right ascension. if you think of the sky as a gigantic clock face it all makes sense. Since there are 24 hours in a day and obviously you can only see 12 hours of the sky at any given time of day [horizon to horizon]. The sun is at the zenith, as viewed from the equator, at precisely noon on the vernal equinox, and this is labeled 0 hours right ascension, Thus the eastern horizon is at 6 hours right ascension at noon on the vernal equinox.

DonutLord
davenn
Gold Member
so is any object's right ascension value only measured on one day of the year, and then kept constant until the vernal equinox comes around again in a year?

no, the right ascension (and declination) of a galaxy, star, nebula is the same (to a respectable precision) over many years.
The only thing that affects it is "stellar drift". If the RA and DEC changed significantly every year, then all our star maps would be out of date every year.
Star atlases (maps) get updated roughly every 50 years to take the drift into account

Dave

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DonutLord
The sun is at the zenith, as viewed from the equator, at precisely noon on the vernal equinox,
It's the mean year moment that's overhead, not the Sun. The Sun will be 7+ minutes late according to the equation of time.

In the equatorial coordinate system, the vernal equinox can be seen as a point (indeed one of two, as we have also autumnal equinox) on the celestial sphere, where ecliptic (annual apparent path of the Sun across the sky) is intersecting the celestial equator, i.e. directly on zenith as seen from locations at equator. It was chosen as the zero point with coordinates (0, 0) in this system. However due to precession of Earth's rotation axis, the right ascension and declination are very slowly changing, therefore an epoch is usually specified when a position of an object is listed, in catalogue for example. Currently used epoch is designated J2000.0, which means that coordinates listed are such as it would be measured on Jan 1st 2000, at noon GMT. You can than calculate the actual coordinates of the object based on these values and the time elapsed since the reference date/time. Note that the corrections are really small on the human timescalles, something on the order of 0.05 degrees per 5 years.

Bandersnatch
mfb
Mentor
You know Earth's orbit around the Sun. You can calculate its current orientation relative to the vernal equinox, and that allows you to measure the right ascension of things in the sky at any point in time.

sophiecentaur