Starship Asterisk*

Micro Moon over Super Moon

Explanation: Did you see the big, bright, beautiful Full Moon last Wednesday night? That was actually a Micro Moon! On that night, the smallest Full Moon of 2014 reached its full phase only a few hours from lunar apogee, the time of its the most distant point from Earth in the Moon's elliptical orbit. Of course, last year on the night of June 22, a Full Super Moon was near perigee, the closest point in its orbit. The relative apparent size of January 15's Micro Moon is compared to the June 22 Super Moon in the above composite image digitally superimposing telescopic images from Perugia, Italy. The difference in apparent size represents a difference in distance of just under 50,000 kilometers between apogee and perigee, given the Moon's average distance of about 385,000 kilometers. How long do you have to wait to see another Full Micro Moon? Until March 5, 2015, when the lunar full phase will again occur within a few hours of lunar apogee.

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I don't feel hungry, but all i can think of when i see today's APOD, is pizza moon

The orbit of the Moon around the Earth (or the orbit of the Moon around the center of mass of the Earth-Moon system) is elliptical.

Today's APOD is certainly proof of that!

Ann

I thought it would be interesting to combine a couple Moon APODs with todays moon-extrema creation. I used Rick Baldridge's APODs from Jan 18, 2014 (micro moon) and his Mar 10, 2012 moon shot (about 1 arcminute smaller than the Jun 22 super moon). The Lick Observatory FoVs are close enough that the 3% enlargement of the Mar 10 APOD (needed to match the Jun 22 super moon) does not appreciably detract from the realism of what the super moon would like from Rick's vantage point.

Interesting comparison.....AAAUUUUUUOOOOOOHHHHH!!!....Oh....sorry.....just the wolf in me....


:---[===] *

Is alter-ego mooning us

GREAT APOD! This comparison demonstrates completely, just how totally INANE the newspaper stories are, year after year, about "don't miss the Super Moon!"

While I've been an apod fan for well over a decade, I have not previously posted a question - (Because all the photos are so wondrously self-evident and the explanations are so marvellously self-explanatory and correct? perhaps...) But I have one on today's.

Ok, pls bear with me as I am very very far from well educated in the practicalities of astronomy and photomanipulation of astronomical observations, but an aspect of today's pictures puzzle me.
If I try to see replication of features on the moon at the edges of the the 2 images - in the zone where you can see features in both the apogee and perigee shots, at first I don't see any match up. And given the whole deal about "the same face of the moon always faces the earth" (with very slight variations at the edges because of various orbital and rotation mechanics-y stuff that I am aware of but never learned in detail) and also that these pics are both from the the same observation location (or at least nearly so, as the text lists the source as being from Perugia) I would have expected the features in that zone to appear to visibly align very closely (as indeed they do in the Micro and Macro comparison shots from 2012 on the astrosnake.com website that is cited). But then I noted that the features do reappear in both images, it is just that the apogee version is rotated about 45 degrees counterclockwise (pls pardon any inaccuracy here, I'm estimating angles with my fingers held over my laptop screen...) relative to the perigee image.

So. My question: is that apparent rotation intentional (and if so, why?), or accidental, or do I completely misunderstand something crucial (but probably very simple) about the images?

msdjwsun wrote:While I've been an apod fan for well over a decade, I have not previously posted a question - (Because all the photos are so wondrously self-evident and the explanations are so marvellously self-explanatory and correct? perhaps...) But I have one on today's.

Ok, pls bear with me as I am very very far from well educated in the practicalities of astronomy and photomanipulation of astronomical observations, but an aspect of today's pictures puzzle me.
If I try to see replication of features on the moon at the edges of the the 2 images - in the zone where you can see features in both the apogee and perigee shots, at first I don't see any match up. And given the whole deal about "the same face of the moon always faces the earth" (with very slight variations at the edges because of various orbital and rotation mechanics-y stuff that I am aware of but never learned in detail) and also that these pics are both from the the same observation location (or at least nearly so, as the text lists the source as being from Perugia) I would have expected the features in that zone to appear to visibly align very closely (as indeed they do in the Micro and Macro comparison shots from 2012 on the astrosnake.com website that is cited). But then I noted that the features do reappear in both images, it is just that the apogee version is rotated about 45 degrees counterclockwise (pls pardon any inaccuracy here, I'm estimating angles with my fingers held over my laptop screen...) relative to the perigee image.

So. My question: is that apparent rotation intentional (and if so, why?), or accidental, or do I completely misunderstand something crucial (but probably very simple) about the images?

I think they didn't take any particular care regarding the orientation. Having said that, the Moon does librate as it goes around the Earth. Do you have a Macintosh, with Keynote installed? And do you mind downloading a file that (warning!) is about 18 MB? If so, I long ago created Moon.key which (greatly speeded up) shows the Moon's accurate appearance, both size and orientation, as it goes round and round and round the Earth. I made it with Carina Software's most excellent Voyager planetarium program. Here it is: http://henry.pha.jhu.edu/Moon.key (remember, it is a BIG file!)

932 KB image

Lunar Libration

msdjwsun wrote:... an aspect of today's pictures puzzle me.
If I try to see replication of features on the moon at the edges of the the 2 images - in the zone where you can see features in both the apogee and perigee shots, ... I noted that the features do reappear in both images, it is just that the apogee version is rotated about 45 degrees counterclockwise (pls pardon any inaccuracy here, I'm estimating angles with my fingers held over my laptop screen...) relative to the perigee image. ...

Here's an illustration of what you're saying. What I find interesting is that if you compare the red angle between the top pair of features and the yellow angle between the other pair, it's not the same angle. Perhaps I need a libation to understand that libration.
Rob

Ann wrote:The orbit of the Moon around the Earth (or the orbit of the Moon around the center of mass of the Earth-Moon system) is elliptical.

Today's APOD is certainly proof of that!

Today's APOD is certainly proof evidence of that!

And to add more pedantry, a perfect circle is also an ellipse. The different sized moons offer evidence of the degree of eccentricity of its elliptical orbit around Earth.

I had the same question about the apparent rotation, and it took me a few minutes to figure it out. I'm not enough of an expert to say that this fully explains the rotation, but it seems about right (and I welcome feedback on this).

The plane of the Moon's orbit is tilted relative to Earth's equator. So the Moon at perigee could be south of the observer, and at apogee it is north, or vice-versa (it works both ways, depending on where we are in the orbital cycle). The camera needs to be aimed differently to take the two pictures. Now, if the pictures were taken at local midnight, the camera would only need to be tilted up and down between the two shots, so no rotation. But if the pictures are taken at different times, there would be a sideways motion as well. Then, to keep the camera oriented the same relative to the local horizon, would require some rotation.

This doesn't explain the slight difference in angles at different parts of the Moon. Maybe that's just distortion in the pictures, libration, imperfect measurement technique, I don't know.

Chris Peterson wrote:

Ann wrote:The orbit of the Moon around the Earth (or the orbit of the Moon around the center of mass of the Earth-Moon system) is elliptical.

Today's APOD is certainly proof of that!

Today's APOD is certainly proof evidence of that!

And to add more pedantry, a perfect circle is also an ellipse. The different sized moons offer evidence of the degree of eccentricity of its elliptical orbit around Earth.

Thanks for correcting me, Chris. I practically always appreciate it!

Ann

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In the northern hemisphere Mare Crisium
(coordinates: 17.0°N 59.1°E) is:

1) near the top of the moon at moonrise
(near the 11:20 "hour position" of the moon
6 hours before lunar zenith)

2) near the 2:20 "hour position" of the moon
at lunar zenith [i.e., on the Meridian] and

3) to the bottom of the moon at moonset
(near the 5:20 "hour position" of the moon
6 hours after lunar zenith)

All of the moon shots were taken at different times
before lunar zenith and, hence, are rotated differently.

(Note: the inclination of the Moon's orbit can may
also contribute an extra ±5º but is a secondary factor.)

Nice image. And you contributors always bring out much more than I first realize is interesting.

As the Earth and Moon rotate around a center of mass, I assume that when the Moon is full, the Earth is always closest to the Sun (a local minimum, that is). If the Moon is full at apogee, then if I understand this, that must be a time when the Earth is swinging farther from the center of mass as well, so it is pushed closer to the Sun by the maximum amount that this effect ever produces. However, the Earth/Moon's orbit around the Sun is elliptical itself, and I am guessing that the eccentricity of the orbit is such that the regular perihelion of the Earth/Moon pair is a larger effect than the amount of the 28-day swings. Looking at Wikipedia: http://en.wikipedia.org/wiki/Earth's_orbit I read that the Earth/Moon are much closer to the Sun in January, they say that in current times perihelion occurs around Jan 3. But that article is only discussing Earth, not really treating the Earth/Moon system, as far as I can tell. Anyway, they list perihelion at 147,098,290 km versus aphelion at 152,098,232 km, a difference of about 5,000,000 km.

So, here's what I'm thinking. If Moon's apogee occurs right at a full moon, just at Earth/Moon's perihelion, then that is the closest Earth ever gets to the Sun. A micro-moon coinciding with Jan 3 would cause the Earth to get closer to the Sun than any other time. But the Earth is only pushed closer to the Sun by it's distance from the center of mass of the Earth and Moon, so I would guess this is only a small boost to the 5,000,000 km effect of the eccentricity of the overall Earth/Moon system's orbit. As I read further, it appears that the center of mass of Earth/Moon is actually within the Earth. That's a surprise! So, the maximum variation all of this causes in Earth's distance from the Sun is going to be only about 4000 km.

Two questions:

1. Anything grossly incorrect in the above? Let me know.
2. Does the fact that Earth reaches perihelion during winter in the Northern Hemisphere make the North's winters any milder than the Southern Hemisphere's ?

-- Mark

MarkBour wrote:Does the fact that Earth reaches perihelion during winter in the Northern Hemisphere make the North's winters any milder than the Southern Hemisphere's ?

Yes, but not in an easily discernible way. The thing is, it is difficult to directly compare northern and southern hemisphere weather and climate because the hemispheres are so different geographically, in terms of the distribution of ocean and land. That has a much more profound impact than the Earth's orbit.

It would be better to say that when precession moves the perihelion point to the northern hemisphere summer (in about 10,000 years), those summers will be warmer, and the winters cooler, than they currently are. Indeed, the combination of precession of the equinoxes and precession of the apsides are an important part of the Milankovitch cycle observed in long term climate variation.

Just a superior APOD image, overcoming every ordinary simple Moon like this one: LPOD ...

ViliMax wrote:Just a superior APOD image, overcoming every ordinary simple Moon like this one: LPOD ...

That is just a gorgeous picture of the moon!

My question is: How much greater is the tidal force of a super-moon to a micro-moon, (assuming the distance from the sun is the same in both cases)? What does this translate to in average tidal changes,( %-wise)?
Also, if you consider that, as the moon orbits the earth, two zones of deformation move around the planet as well, how much more is the crest deformed by a super moon?

neufer wrote:

http://glenavalon.com/knowth01.html wrote:

[b][color=#0000FF][size=150]The moon rotates 360° over a ~25 hour time period :!: [/size][/color][/b]

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.


In the northern hemisphere Mare Crisium
(coordinates: 17.0°N 59.1°E) is:

1) near the top of the moon at moonrise
(near the 11:20 "hour position" of the moon
6 hours before lunar zenith)

2) near the 2:20 "hour position" of the moon
at lunar zenith [i.e., on the Meridian] and

3) to the bottom of the moon at moonset
(near the 5:20 "hour position" of the moon
6 hours after lunar zenith)

All of the moon shots were taken at different times
before lunar zenith and, hence, are rotated differently.

(Note: the inclination of the Moon's orbit can may
also contribute an extra ±5º but is a secondary factor.)

Thanks Neufer, this is one of the most surprising phenomena when observing the Moon over the course of a night, just how much the orientation of the features on the face of the Moon changes hour by hour. If you stand outside and look at the full Moon, and remember that the Moon rises in the east and sets in the west because the Earth is rotating in the opposite direction (west to east), as you see the Moon appearing to rotate in the sky, you're actually seeing evidence of the Earth's rotation relative to the Moon. There have been moments when I've been able to feel the Earth moving, or at least to imagine that physical sensation. And you don't need a telescope, a camera, or a computer to observe this effect, just some free time, careful observation, and a chance to sleep in the next day. A series of quick sketches can be a useful memory aid.

On another note, I'm always surprised by pictures illustrating the difference between the apparent sizes of the Moon at apogee and perigee. Watching the Moon from one night to the next, the change is subtle, and the Moon always looks Moon-sized to me.

According to the moon's data of the nssdc (http://nssdc.gsfc.nasa.gov/planetary/fa ... nfact.html), it's distance at perigee is 363300 km, not the 356790 km reported on Anthony Ayiomamitis's web site. Since the moon at apogee is at 405500 km from earth, I think the difference between these positions should be 42000 km, rounded to 40000 km, not 50000 km.

Donald Pelletier wrote:
According to the moon's data of the nssdc (http://nssdc.gsfc.nasa.gov/planetary/fa ... nfact.html), it's distance at perigee is 363300 km, not the 356790 km reported on Anthony Ayiomamitis's web site. Since the moon at apogee is at 405500 km from earth, I think the difference between these positions should be 42000 km, rounded to 40000 km, not 50000 km.

The moon's perigee & apogee are not fixed numbers;
both vary by thousands of kilometers thanks to strong solar tidal forces.

neufer wrote:

Donald Pelletier wrote:
According to the moon's data of the nssdc (http://nssdc.gsfc.nasa.gov/planetary/fa ... nfact.html), it's distance at perigee is 363300 km, not the 356790 km reported on Anthony Ayiomamitis's web site. Since the moon at apogee is at 405500 km from earth, I think the difference between these positions should be 42000 km, rounded to 40000 km, not 50000 km.

The moon's perigee & apogee are not fixed numbers;
both vary by ~ 500 km each thanks to strong solar tidal forces.

http://fourmilab.ch/earthview/pacalc.html

Perigees vary much more dramatically than apogees.

Anthony Barreiro wrote:
Perigees vary much more dramatically than apogees.

My bad.