Starship Asterisk*

A Total Lunar Eclipse Video

Explanation: Tonight a bright full Moon will fade to red. Tonight's moon will be particularly bright because it is reaching its fully lit phase when it is relatively close to the Earth in its elliptical orbit. In fact, by some measures of size and brightness, tonight's full Moon is designated a supermoon, although perhaps the "super" is overstated because it will be only a few percent larger and brighter than the average full Moon. However, our Moon will fade to a dim red because it will also undergo a total lunar eclipse -- an episode when the Moon becomes completely engulfed in Earth's shadow. The faint red color results from blue sunlight being more strongly scattered away by the Earth's atmosphere. A January full moon, like the one visible tonight, is referred to as a Wolf Moon in some cultures. Tonight's supermoon total eclipse will last over an hour and be best visible from North and South America after sunset. The featured time-lapse video shows the last total lunar eclipse -- which occurred in 2018 July. The next total lunar eclipse will occur only in 2021 May.

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Thanks for the alert and the illustrative video. I'm hoping for clear skies in central Illinois. It's been cloudy a lot, but the forecast is clear.

In the third portion of the video, "Capture from TheSkyX", the umbra appears to be moving in an intriguing pattern against the background of fixed stars. I think TheSkyX is software for controlling a telescope, so I wonder at the movement of the shadow.

I think the "intriguing pattern" is formed by virtue of the observer's location on the surface of a spinning Earth over the course of the two or three days straddling the eclipse. That is, it is topocentric, not geocentric.

Nicely done.

Nice video. Something puzzles me though about the scientific laws in the 3rd video illustration. I assume the video is sped up as the moon just zips by. I understand that. But what about the stars in the background? If you watch the time stamp at the top of the screen the total eclipse starts just after midnight July 28, 2018. The eclipse ends around 5:30 am and the moon shoots off the left edge of the screen around 9 am. Now unless this was shot at the South Pole, I know that where I live at 9 am in July that the sun has already risen, the sky is blue, and you can't see any of the night stars in the sky any longer. Where is the Sun? Even stranger the time clock actually starts at 10 am on July 26 with hundreds of stars visible in the sky. The Sun didn't rise on the 26 or on the 27. It must have been at the South Pole. But isn't the Moon upside down at the South Pole?

De58te wrote: ↑Sun Jan 20, 2019 2:22 pm Nice video. Something puzzles me though about the scientific laws in the 3rd video illustration. I assume the video is sped up as the moon just zips by. I understand that. But what about the stars in the background? If you watch the time stamp at the top of the screen the total eclipse starts just after midnight July 28, 2018. The eclipse ends around 5:30 am and the moon shoots off the left edge of the screen around 9 am. Now unless this was shot at the South Pole, I know that where I live at 9 am in July that the sun has already risen, the sky is blue, and you can't see any of the night stars in the sky any longer. Where is the Sun? Even stranger the time clock actually starts at 10 am on July 26 with hundreds of stars visible in the sky. The Sun didn't rise on the 26 or on the 27. It must have been at the South Pole. But isn't the Moon upside down at the South Pole?

It's a sky simulator. Normally, your view is from a transparent Earth (that is, you can see things below the horizon) and no atmosphere (so no extinction or blue sky). You don't see the Sun because it's never in this particular frame during the simulation (if it were, it would just be like another star going by, although drawn at the size of the Moon). The time reflects the timezone the simulator is set for (usually the timezone of the computer itself). In this case, the eclipse was maximum at UT 2018.07.25 20:22, so we're seeing the simulation run at UT+6, consistent with west central Asia.

Nitpicker wrote: ↑Sun Jan 20, 2019 9:46 am I think the "intriguing pattern" is formed by virtue of the observer's location on the surface of a spinning Earth over the course of the two or three days straddling the eclipse. That is, it is topocentric, not geocentric.

In other words, we're seeing the diurnal parallax of earth's shadow.

BobStein-VisiBone wrote: ↑Sun Jan 20, 2019 4:36 pm

Nitpicker wrote: ↑Sun Jan 20, 2019 9:46 am I think the "intriguing pattern" is formed by virtue of the observer's location on the surface of a spinning Earth over the course of the two or three days straddling the eclipse. That is, it is topocentric, not geocentric.

In other words, we're seeing the diurnal parallax of earth's shadow.

Thank you, Nitpicker! It makes sense, now. And thank you BobStein for pointing me to a helpful follow-up article and the name for this apparent motion.

So the Moon's apparent motion must also be subject to these shifts. In the video sequence, it is not very apparent for the Moon. Since the Moon is also moving rapidly by in its orbit, I guess the effect is just a slowing and a speeding of the Moon's progress, which is hard for me to observe in the simulated video, but I think I can see it after watching it several times carefully.

Cloudy and wet...will probably miss it...

:---[===] *

MarkBour wrote: ↑Sun Jan 20, 2019 8:51 pmSo the Moon's apparent motion must also be subject to these shifts.

Most welcome MarkBour. Yes you're right there must be a diurnal parallax of the moon too.

Earth's umbra is a cone 870K miles long (1.4 gigameters) that every nighttime we slip inside of. I'm guessing the brownish circle in the video is a slice of that cone at the distance of the moon, 240K miles (0.38 gm) from Earth. So I think shadow and moon should both parallax to the same degree.

But diurnal parallax of the moon should appear to us as only a slight daily slowing and speeding of the moon's apparent motion against the stars, which averages a screaming 360 degrees per month. Diurnal parallax of Earth's shadow on the other hand averages a turtly 360 degrees per year, which is so much slower that its parallax should be much more obvious, with retrograde and everything.

A related phenomenon is diurnal libration which makes the moon seem to rotate back-and-forth a tiny bit each day and give an Earth observer a tiny peek at east and west limbs. I get the impression there's a greater monthly libration due to the eccentricity and tilt of lunar orbit.

The apparent daily loop in the path of Earth's shadow against the stars, is only so obvious because someone worked it out in topocentric coordinates and someone else was nice enough to create a video of the resulting projection of the full shadow at the Moon's distance, which is otherwise (obviously) not observable.

Many backyard, motorised tracking telescopes, allow tracking at a solar, lunar, or (default) sidereal rate. They are all slightly different, but constant tracking rates, even though the true solar and lunar rates are never quite constant. More expensive/accurate scopes account for non-uniform rates.

BobStein-VisiBone wrote: ↑Mon Jan 21, 2019 12:46 am

MarkBour wrote: ↑Sun Jan 20, 2019 8:51 pmSo the Moon's apparent motion must also be subject to these shifts.

Most welcome MarkBour. Yes you're right there must be a diurnal parallax of the moon too.

Earth's umbra is a cone 870K miles long (1.4 gigameters) that every nighttime we slip inside of. I'm guessing the brownish circle in the video is a slice of that cone at the distance of the moon, 240K miles (0.38 gm) from Earth. So I think shadow and moon should both parallax to the same degree.

But diurnal parallax of the moon should appear to us as only a slight daily slowing and speeding of the moon's apparent motion against the stars, which averages a screaming 360 degrees per month. Diurnal parallax of Earth's shadow on the other hand averages a turtly 360 degrees per year, which is so much slower that its parallax should be much more obvious, with retrograde and everything.

If you could see it, the daily parallax of the Earth's shadow (antisolar point) would look similar to the Moon's except the angular magnitude is down ratio of the Sun's and Moon's orbital radii ( about 390). Instead of the moon's ~1° RA and ~½° Dec peak to valley amplitude variations, the antisolar point p-v position varies by ~10" RA and ~5" Dec. Just like the Moon, the antisolar point would appear to slow down and speed up, but it is much less obvious than the moons oscillations. For the Moon, the ratio of daily p-v RA travel to the daily orbital travel is roughly 1°÷ 12°or about 10%. For the antisolar point RA oscillations, the ratio is roughly 10" ÷ 1°~ 0.3%. I'd say these oscillations are far less obvious. It would be like at the todays video with 30x smaller shadow oscillations amplitude.
Note You get this comparison ratio simply by: [30days÷365 days] x [150,000,000km÷384,000km] = 32x

Hi alter-ego,

You've confused me now. Are you saying that the last third of the APOD video is showing oscillations in the Earth's shadow (at Moon distance) that are much larger than they should be?

Thinking more about this (still without working it all out completely), I think the antisolar point may be somewhat independent of the projection of the Earth's shadow at the Moon's distance.

The antisolar point projects to infinity from the centre of the Sun, through the observer (does it not?). At infinity, it should not exhibit any parallax.

However, the projection of the shadow at the Moon's distance should exhibit the same diurnal parallax displacement as the Moon, I think. But unlike the Moon, the shadow's apparent topocentric motion against the stars forms retrograde loops (assuming the observer is far enough away from the Earth's poles), because the geocentric motion of the Sun is ~12 times slower than the geocentric motion of the Moon.

Nitpicker wrote: ↑Mon Jan 21, 2019 1:33 am The apparent daily loop in the path of Earth's shadow against the stars, is only so obvious because someone worked it out in topocentric coordinates and someone else was nice enough to create a video of the resulting projection of the full shadow at the Moon's distance, which is otherwise (obviously) not observable.

Right. So, I'm all for sending up a large white sheet (an IMAX screen) and spending lots of fuel to get it to hold its shape and position right there.

Then every night, much more faithfully than the Moon, we would be able to observe the anti-moon.

I think the Flat Earth Society would love it.

(Most of this image is thanks to The Sky LIVE https://theskylive.com/.)

Hey Nitpicker, it's good to hear from you again. I don't often visit the other forums so maybe I've just missed your posts. Anyway, long story short, I was totally missing the discussion (dim bulb last night I guess). I have just a few comments:

Nitpicker wrote: ↑Mon Jan 21, 2019 10:56 pm Thinking more about this (still without working it all out completely), I think the antisolar point may be somewhat independent of the projection of the umbra at the Moon's distance.

Yes, that's true, and I confused myself by using the antisolar point here. You're right, the antisolar point is totally observer dependent and, in practice, independent of the Earth's shadow.

The antisolar point projects to infinity from the centre of the Sun, through the observer (does it not?). At infinity, it should not exhibit any parallax.

True definition, but except for the Earth's poles, the Sun does display a varying parallax in the course of a day to any observer. Therefore the observer's antisolar point also changes by the same angle, i.e. as viewed from the sun, the observer's position on the celestial sphere does change by the parallax as the Earth rotates. With all that said, it's clear the umbral center is the antisolar point for only a geocentric observer where the parallax is zero.

However, the projection of the umbra at the Moon's distance should exhibit the same diurnal parallax displacement as the Moon, I think. But unlike the Moon, the umbra's apparent topocentric motion against the stars forms retrograde loops (assuming the observer is far enough away from the Earth's poles), because the geocentric motion of the Sun is ~12 times slower than the geocentric motion of the Moon.

Yes, both Moon and Earth's shadow (viewed on the Moon) show the same daily parallax motion. Considering motion in right ascension, the threshold lunar orbital period for retrograde loops to begin (when the moon appears to just stop moving and doesn't reverse) is 3x longer, or the Sun's geocentric motion is ~4x slower than that of the Moon.

Silly me, of course the antisolar point must change corresponding to the parallax of the Sun.

And no, I've been nought but an observer on these pages for a while. Good to be back, however long it lasts.

...

MarkBour, I just read an article the other day about a new proposal for billboards in space. Your "anti-moon" idea gave me the same feeling. Barf.

Nitpicker wrote: ↑Tue Jan 22, 2019 7:04 am ...
MarkBour, I just read an article the other day about a new proposal for billboards in space. Your "anti-moon" idea gave me the same feeling. Barf.

Yes, me too, actually.