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Comment by skfjeofh | Hacker News Reader

The article is missing the most disappointing fact of all, though.

If there was a whole 24-hour period during which the sun was on the celestial equator, then 12 hours would be day and 12 night (going by the time the centre of the sun's disc rose and set).

But the sun is always moving along the ecliptic, so the day we call the equinox is just the 24hr period within which the sun crosses the equator, which is an instantaneous event.

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> On Saturday, September 23, at 6:50 A.M. UTC (2:50 A.M. EDT or 11:50 P.M. Friday PDT), the sun will be directly over Earth’s equator, which is how astronomers define the equinox.

Article starts out by specifying the time of the event

As has been noted in another comment, the article makes that statement, but skips over the implications of the constant apparent motion of the sun for the length of day.

It gives the impression that, geometrically speaking, there is a day on (or near) the equinox which has equal darkness and daylight. Of course, this isn't guaranteed at all, and certainly not for every point on the earth's surface.

nit: The sun is not moving along the ecliptic or crossing the equator. It is the Earth that is actually doing all the moving.

One can choose to pin a frame of reference to the Earth, it just won't be an inertial one (and quite noticeably not.)

Perhaps you would accept phrasing the sentence in question as saying the Earth-Sun axis is doing these two things?

The Sun and Earth orbit around a central point between the two bodies, which happens to be very close to the center of the sun. But that doesn't change the fact that they are both orbiting around each other.

This is somehow more wrong than what you answered, which was already very wrong. (Wrong in an aesthetic sense, since both are ill-defined in physics, and only carry aesthetic value in Engilsh.)

The point is neither central, nor solely determined by the earth and sun. The moon and Jupiter influence the orbits of the earth and sun as well (of course as do all the other bodies to a much smaller degree)

Depends on the inertial frame, the sun is flying through our galaxy :)

If I search for ‘ecliptic’, I get 2 different definitions: one is the “apparent path of the sun” (where the observer is the reference point), and the other is the plane of earth’s orbit around the sun (where the sun is the reference point). A good portion of the time, the plane of earth’s orbit is referred to as the ‘ecliptic plane’ and not just ‘ecliptic’. Maybe using ‘plane’ as a qualifier is a nice way to distinguish these ideas. Google’s chosen quick-definition, the one where it shows you the result without linking to a website, is the apparent-path definition.

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