Is it possible for a lunar eclipse to occur before sunset? | Hacker News Reader

Is it possible for a lunar eclipse to occur before sunset?

astronomy.stackexchange.com

137 pointsgeorgecmu30 comments

Back in Classical Greece there was some debate as to what the cause of a lunar eclipse was. By this time it was generally accepted that it was due to the Earth coming between the Moon and the Sun and casting a shadow. But one point of evidence that was presented against this theory was that on occasion lunar eclipses were observed at sunset. You could clearly see both the Sun and the Moon, so the Earth did not appear to be between the two. This issue with the idea that lunar eclipses are due to the Earth's shadow wasn't really satisfactorily addressed until the 1700s or so when the refraction of light was systematically studied (though by the Hellenistic Era no one seriously doubted it).

This actually wasn't the only time when the refraction of the Earth's atmosphere produced surprising observations in the ancient world. One of the devices the Greeks used to determine the time of an equinox was what is called an equatorial ring. It's basically just a metal hoop oriented at an angle equal to 90 degrees minus the latitude. (Here's a diagram: https://en.wikipedia.org/wiki/Equatorial_ring#/media/File:Eq...) During an equinox the Sun would be right on the celestial equator, and the shadow cast by the upper half of the hoop would fall exactly on the bottom half.

But it was noticed that sometimes, the shadow would fall on the bottom half early in the morning, then cross to the other side, cross back, and then later cross over once again --- it seemed that there was three equinoxes! Ptolemy explained this away as being due to instabilities in the device's orientation --- somehow it was very slightly tilting and so it wasn't possible to measure the time of the equinox to better than half a day or so. But today we know that this was also due to the refraction of the atmosphere. Early in the morning, the Sun would be refracted to appear to be just above the celestial equator, but as it rose, the refraction would lessen and it would fall back to its original place. Then later in the day it would cross the celestial equator "for real" when the equinox actually occurred. So the device was much more accurate than ancient astronomers gave it credit for!

Besides the refraction, there's also a geometrical effect. An observer is not on the line of the Sun-Earth-Moon centers; he's offset from it by Earth's radius at Earth's surface, so he's at one point of a triangle and sees a Sun-Moon angle of slightly less than 180º.

The refraction is a larger effect, but even an airless planet could have an observer seeing the centers of both the Sun and Moon at eclipse time just above the horizon.

That is interesting… but to my intuition, it seems like that would have the opposite effect? You are offset in the “wrong” direction, ie the sun and the moon are further “down” and more obscured by the earth? I might be thinking all wrong about this, though.

I’d also question if this can have any meaningful effect on the sun, which is about 23500 Earth radii away. That’s a very, very slight perspective shift.

Imagine an ant standing on some random point on a soccer ball in the middle of a field. Now rotate the ball so where the ant is will be directly facing the side of the field, 90 degrees rotation of the ball from facing either goal. At this specific point the ant can see each goal at each end of the field even though the ball is directly between the middle of the two goals. Well, depending how tall the ant is given the goals are so close to the ball.

A huge distance is actually a helping factor in this case. A 1.75 meter person can see 4,700 away before the horizon curves out of view. That means (ignoring the bending of light by the atmosphere still) the viewing angle allows for 1 meter "down" every ~2,685.7 meters out. The radius of the Earth is ~6,371,000 meters so to be able to see the middle of an object from ~the radius of the earth "down" with that viewing angle would require the object to be a minimum of ~2,685.7 * 6,371,000 = 17,110,594,700 meters away. That's a minimum distance of ~0.11 AU so the person would only get a partial view of the moon but a full view of the sun because the moon -isn't- far enough away! Well technically the Earth could rotate slightly farther while having the middle of the sun still visible but I'm too lazy to calculate if it's actually enough, my hunch is no. If you wanted to see the middle of the moon and the middle of the sun (with no bending by the atmosphere) during an eclipse you'd have to be near the top of Everest while it happens to be exactly rotated between each.

So are you saying that the offset of 1 Earth/soccer ball radius is helping or hurting? It’s still seems to me like it’s just getting in the way. (Though I guess the ant wouldn’t see much of anything if it was down in the grass.)

It helps you see things either side of the ball if you're offset from the centre.

Forgive the hasty diagram but https://imgur.com/a/aoZIZTs

The Earth is entirely in-between the wildly out of scale Sun and the Moon, but a giant standing on the "top" of the Earth can see both.

That's the canonical version of the story.

I wonder if anyone back then, say an armchair astronomer, tried to argue that it was due to the atmosphere, and backed their argument up by comparing the atmosphere to how a pool of water bends light.

We'll never know.

Interestingly enough, modern Greece - so presumably also ancient Greece but I'm not sure - are one of the very few places where the water can be so still and so clear as to appear absolutely transparent. I actually experienced this last October in Greece and I never imaged that seawater could be clearer than bottled spring water.

See also the green flash: https://en.m.wikipedia.org/wiki/Green_flash

Wikipedia has a picture from 2014: https://en.wikipedia.org/wiki/File:Lunar_eclipse_at_sunrise_...

Here's a nice post thinking through how you might actually observe this: https://photoephemeris.com/articles/the-selenelion-challenge

Sunset is kind of a fluid thing. The lunar eclipse on May 15 last year happened right around sunset as viewed in the SF bay area. A little way further south or west would have been an even brighter sky for the eclipse. https://photos.app.goo.gl/pjbefmFcv2THZEQ19 are a few photos I took of this. The red of the eclipse was initially almost invisible due to how much light there still was in the sky, and it only became clearer as the ambient light diminished.

Yes, a selenelion will happen just before sunset or just after sunrise when the sun and moon are at opposite points of the horizon. This is also called a horizontal eclipse and is due to the atmosphere not the planet. [1] [2]

[1] https://sciencenotes.org/selenelion-eclipse/

[2] https://www.space.com/27338-total-lunar-eclipse-rare-sunrise...

It is possible (atmospheric effects as stated by others/the article), but not in a useful sense of having something that looks impressively like a lunar eclipse.

In other trivia, expect that a solar eclipse will also happen somewhere on Earth in any month that has a lunar eclipse. (exactly because of similar alignment issues)

Isn't that only true the other way around? The earth is bigger than the moon, so I'd think sometimes you'd get a total lunar eclipse when there's only a partial corresponding solar eclipse, and diddly when there's only a partial lunar eclipse.

Oh, I don't mean exactly the same magnitude of eclipse -- just that whether annular/partial/etc, if there is a lunar eclipse, there will generally be a solar eclipse (of any type).

A lunar eclipse can happen at any time of the day, might only be visible on the other side of the planet - maybe the question he meant to ask was "can you see ...."

Every lunar eclipse happens at every possible time of the day, simultaneously.

And all lunar eclipses, except possibly the last one, precede at least one sunset.

Hrm...

The Moon's orbit will decay to the point that solar eclipses are no longer possible in about 600 million years. The Sun will continue to rise and set following that point, in all likelihood.

This is however about the same time that the carbon-silicate cycle will have been disrupted to the point that C3 photosynthesis is no longer possible. Humans (or their descendant species) may not be observing either lunar or solar eclipses by that time, at least not from Earth.

And by 1.5--4.5 billion years from now, still within the main solar sequence, the Moon will have drifted far enough from the Earth that the Earth's axis is no longer stabilised by the Moon. By this point, even lunar eclipses might no longer be possible.

(The other factor is that the Moon drifts so far out that it orbits outside the Earth's umbra, such that total lunar eclipses no longer occur, though "blood moon" penumbral eclipses could still happen. I don't know what the timeline for this might be. But again, so long as the Earth itself rotates and isn't consumed by the Sun, sunsets will still occur.)

By 800 my to 1.6 by, solar luminosity will have increased to the point that life on Earth's surface is likely impossible.

Wikipedia, "Timeline of the Far Future": <https://en.wikipedia.org/wiki/Timeline_of_the_far_future>

Sidenote: Past events are designated using the abbreviations kya, mya, bya, etc., (thousand, million, billion years ago). I'm not aware of a standard abbreviation for time in the future, though it would seem kyf, myf, byf, etc., might be reasonably used for this. Or perhaps "kyp" or "kyn" (thousand years from present, thousand years from now).

Seems that "FY" has at least some support: <https://www.abbreviations.com/abbreviation/future%20years>

Surely that would only be true if we didn't have time zones ....

Yes always possible. If you consider the light spectrum is split during sun set and rise. So you will get more red light spectrum, during set/rise. You will see more green during night ie Nothern lights. And obviously mostly blue during the day.

The green hue of the northern lights is not caused by refraction. It’s caused by oxygen molecules giving off light after interacting with charged particles from the solar wind (which is being directed by the earths magnetic field into the atmosphere at northern latitudes.) Oxygen gives off photons in a greenish hue when the excited electrons return to their ground state, while nitrogen gives off a bluer hue, etc.

i just realized you explained the red dawn and red dusk in Arizona, far more scientifically than the urban legend that it's "pollution from California".

It is mostly caused by particulate matter in the air. Not so much pollution from California, though I'm sure a bit of it is. More likely to be dust and our own pollution.

is there a citation for that?

the reason i ask is the parent comment talks about the scattering of light at sharp incidence levels favoring certain wavelengths, with the reds being what naturally is dominant at dusk and dawn light-hitting-atmosphere angles.

https://www.metoffice.gov.uk/weather/learn-about/weather/opt...

Looked through a few pages, this one I think explains it fairly well https://demos.smu.ca/index.php/demos/astronomy/35-red-sunset...

The answer it gives is more of a combination of what I thought and what the parent said, tl;dr version: scattering both due to just normal oxygen and nitrogen composition of the air and also pollutants (which I think dust would also qualify, though most things I could find that specified were talking more about sulfur compounds).

Neat!

Betteridge says no, but the headline says yes.

https://en.wikipedia.org/wiki/Betteridge%27s_law_of_headline...

Perhaps there should be an exception for Stackoverflow.

This is the difference between a title and a headline.

Reply on news.ycombinator.com