<p>The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.</p>

<p>Given the various ways that objects in orbit over Earth are perturbed, any set of keplerian orbital parameters that describe the orbit of any object over the Earth (including the Moon!) will need updating long before the drift of the Vernal Equinox along the celestial equator becomes significant.</p>

CC BY-SA 4.0
2020-04-17T22:08:30.053Z
2020-04-17T22:08:30.053Z
notovny
<p>I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is <em>always</em> <em>on</em> the equator. And I suppose it <em>usually</em> is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?</p>

<orbital-mechanics><orbital-elements><right-ascension><equinox>
Is the vernal equinox always in zenith somewhere on the equator?
OZ1SEJ
# Is the vernal equinox always in zenith somewhere on the equator?

(Asked by: username_0 on 2020-04-17)

I'm trying to visualize the satellit orbital parameter "right ascension of the ascending node" (RAAN). Measuring the angle from the vernal equinox to the point where the satellite's orbit crosses equator from south to north makes sense, as long as the vernal equinox is *always* *on* the equator. And I suppose it *usually* is. But the Earth's axis precesses--how can the RAAN make any sense, if the vernal equinox is no longer directly above the Earth's equator?

**Tags:** orbital-mechanics, orbital-elements, right-ascension, equinox

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## 1 answers

### Answer by username_1 on 2020-04-17. Score: 4 | Accepted answer

The Vernal Equinox is defined by the point where the sun's path across the sky, the Ecliptic, crosses the Celestial Equator. It is always going to be on the Celestial Equator.

Given the various ways that objects in orbit over Earth are perturbed, any set of keplerian orbital parameters that describe the orbit of any object over the Earth (including the Moon!) will need updating long before the drift of the Vernal Equinox along the celestial equator becomes significant.


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