<p>The local weatherman declared that fall starts at exactly 10:21am (local time). Ignoring the fact that seasonal boundaries have multiple definitions (meteorological, astronomical, etc.), what he perhaps means to have said is that the fall equinox occurs at that moment.</p>

<p>Presumably, that is the moment at which Earth's axis of rotation is exactly perpendicular to the Sun-Earth line.</p>

<p>My question is: to what degree of precision can this be measured? The weatherman's statement was to the minute. But is it actually measured to a finer precision? If so, plus or minus how many seconds? </p>

<p>What about the the solstices? Can they be measured to the same degree of accuracy? Are there factors which uniquely affect the accuracy of either measurement?</p>

CC BY-SA 3.0
2016-09-20T16:08:32.943Z
Earth's axis is never perpendicular to its orbital plane, but at the equinoxes, the Sun crosses Earth's equatorial plane, which is perpendicular to its axis.
Mike G
CC BY-SA 3.0
2016-09-27T23:32:16.887Z
@barrycarter Yes, the Earth's axis is at a fixed orientation in space (if you ignore precession, nutation, etc.), which is inclined relative to the orbital plane. But twice a year, it does become perpendicular to a line drawn between the the centers of Sun and Earth (remember, it's the Sun-Earth line that is moving, not the Earth's axis)
Anthony X
CC BY-SA 3.0
2016-09-28T00:56:22.087Z
@barrycarter You have to look at it from the correct frame of reference.
Anthony X
<earth>
Measurement precision of celestial events
Anthony X
# Measurement precision of celestial events

(Asked by: username_0 on 2016-09-20)

The local weatherman declared that fall starts at exactly 10:21am (local time). Ignoring the fact that seasonal boundaries have multiple definitions (meteorological, astronomical, etc.), what he perhaps means to have said is that the fall equinox occurs at that moment.

Presumably, that is the moment at which Earth's axis of rotation is exactly perpendicular to the Sun-Earth line.

My question is: to what degree of precision can this be measured? The weatherman's statement was to the minute. But is it actually measured to a finer precision? If so, plus or minus how many seconds?

What about the the solstices? Can they be measured to the same degree of accuracy? Are there factors which uniquely affect the accuracy of either measurement?

**Tags:** earth

> **Comments:**
> **username_1**: Earth's axis is never perpendicular to its orbital plane, but at the equinoxes, the Sun crosses Earth's equatorial plane, which is perpendicular to its axis.  
> **username_0**: @barrycarter Yes, the Earth's axis is at a fixed orientation in space (if you ignore precession, nutation, etc.), which is inclined relative to the orbital plane. But twice a year, it does become perpendicular to a line drawn between the the centers of Sun and Earth (remember, it's the Sun-Earth line that is moving, not the Earth's axis)  
> **username_0**: @barrycarter You have to look at it from the correct frame of reference.  

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