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Comment by Waterluvian | Hacker News Reader

Okay so it is what I think it is. Equal day and night. Just with a few equally fascinating pedantic details that make it not quite exactly true.

I could go for more articles like this. In the little details that usually don’t matter are motes of fascination.

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I was relieved it was what I thought it was, too. Learning equinox means 'equal night', and solstice means 'sun still' will remain stuck in my head for the rest of my life, though.

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.

> 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.

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)

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.)

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.

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

It seems I forgot enough Latin to not be misled.

I agree with that, but I do not like the tone of "no you are actually wrong" from the article, in the way that a scientist could be explaining to you that a tomato is a fruit all day long and you'd still eat it with your salad.

This sort of 'conversational journalism' has become endemic, and it's terribly obnoxious. Scientific American used to be a quality publication, but their editorial standards have sunk to the lowest common denominator over the last 2 decades.

Journalism lost over 90% of its funding after classifieds were gone and tech companies gobbled up all the advertising money.

The current low quality is a natural consequence of the economic incentives.

Speaking of pedantic details, I'm surprised it doesn't mention how the fact that Earth's orbit is an ellipse rather than a circle messes up the dates quite significantly (it's not February's fault).

I expect the same effect on a smaller scale would also mess up "equal day and night".

It’s all Kepler’s fault.

That Copper Knickers guy has a lot to answer for too!

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