APOD and General Astronomy Discussion Forum
https://asterisk.apod.com/
It's good to be reassured that I'm not the only person who sometimes makes little misteaks.neufer wrote:My bad.Anthony Barreiro wrote:
Perigees vary much more dramatically than apogees.
The main reason for the different angle is that, because of libration, the centre of the circle of the Full Moon which appears to us each month, is not the same point on the Moon each month. In other words, the coordinates (0N, 0E) appear to drift around from the centre of the circle of the illuminated Full Moon.rstevenson wrote: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.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. ...
Rob
Hi Art. The apparent tip or "rotation wrt one's horizon is not that simple to be generalized for everyone.
See the Sun vs. Super Moon APOD. The difference is smaller. The aphelion (Micro) Sun is just over 3% smaller than the perihelion (Super) Sun. Using the deceptively descriptive vernacular, the "Super" Moon is bigger than the "Super Sun."MarkBour wrote:Are there any similar images (perhaps previous APODs) that show the maximum-sized and minimum-sized Sun? Is the relative difference less?
Being visible and accounting for all, or most of, the clockwise/ counterclockwise rotation that is seen in both the APOD and your great as overlay are two different things.alter-ego wrote:
Hi Art. The apparent tip or "rotation wrt one's horizon is not that simple to be generalized for everyone.
The moon certainly rocks over 24 hours for most locations, but the full 360°rotation occurs only at the lower latitudes where the moon reaches the zenith / nadir. The latitude limits for the full flip is ≈ ±29°. Of course easiest case to consider are earth's poles. When the moon is visible it is visible all day and there is no 24-hour periodic rocking. However, the monthly rocking due to libration is visible.
However, people tend to fool themselves about what "Moon-sized" is. Its brightness fools us into thinking it's much larger, leading to surprise when we see that it's actually smaller than "small" things like the Pleiades or Orion nebula.Anthony Barreiro wrote: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.
Yes, this is also true. The Moon is a most surprising object of contemplation. So near, so familiar, and yet so mysterious ... .Chris Peterson wrote:However, people tend to fool themselves about what "Moon-sized" is. Its brightness fools us into thinking it's much larger, leading to surprise when we see that it's actually smaller than "small" things like the Pleiades or Orion nebula.Anthony Barreiro wrote: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.
Of course, the apparent rotation in the sky is about the axis defined by the center of the Moon and the viewer. Libration- which isn't real motion at all- is approximately (but not exactly) about the Moon's axis of rotation.Nitpicker wrote:From most habitable places on Earth, the Moon only appears to rotate from moonrise to moonset, becuase we choose to look at it relative to our local horizon. If we instead look at the Moon in celestial coordinates, by aligning our necks with the Earth's axis of rotation -- i.e. with an equatorially mounted head -- the Moon does not appear to rotate. But it still librates, regardless of how we look at it.
I hope you will change your avatar to illustrate your equatorially mounted head. I want to see how you've done that. I already know the angular width of your thumb at arm's length.Nitpicker wrote:... aligning our necks with the Earth's axis of rotation -- i.e. with an equatorially mounted head ... .
Libration is real relative motion, resulting mainly from the variable orbital speed of the Moon and its constant axial rotation rate. Libration has three main components: longitudinal, latitudinal and diurnal. Longitudinal libration is largest component, and is about the Moon's axis of rotation. But the other components are quite significant, too, in proportion to the entire effect.Chris Peterson wrote:Libration- which isn't real motion at all- is approximately (but not exactly) about the Moon's axis of rotation.
I'll just describe it instead. It's very easy. In the Northern Hemisphere (only above the tropics for back safety), when the Moon is up and the sky is clear, go outside, stand up straight and face South. Now lean back until the top of your head points to Polaris. Hold that backwards lean. Now, relative to your lean, rotate your head left/right until you reach the Right Ascension of the Moon. Then tilt your head up/down until you reach the Declination of the Moon. If doing this for prolonged periods, you will need to slowly rotate your head to the right, in order to track the Moon accurately.Anthony Barreiro wrote:I hope you will change your avatar to illustrate your equatorially mounted head. I want to see how you've done that. I already know the angular width of your thumb at arm's length.Nitpicker wrote:... aligning our necks with the Earth's axis of rotation -- i.e. with an equatorially mounted head ... .
What if one has a Dobsonian mounted headNitpicker wrote:
From most habitable places on Earth, the Moon only appears to rotate from moonrise to moonset, because we choose to look at it relative to our local horizon. If we instead look at the Moon in celestial coordinates, by aligning our necks with the Earth's axis of rotation -- i.e. with an equatorially mounted head -- the Moon does not appear to rotate.
(; e.g., like an owl.)LIBRATE, intransitive verb (L libratus, past part. of librare, fr. libra balance)Nitpicker wrote:
But it still librates, regardless of how we look at it.
Libration is not real motion. It's an illusion created by our viewpoint, non-circular and tilted orbits. The Moon is rotating uniformly. An accelerometer on it wouldn't show any of the oscillating motion we call libration.Nitpicker wrote:Libration is real relative motion, resulting mainly from the variable orbital speed of the Moon and its constant axial rotation rate. Libration has three main components: longitudinal, latitudinal and diurnal. Longitudinal libration is largest component, and is about the Moon's axis of rotation. But the other components are quite significant, too, in proportion to the entire effect.Chris Peterson wrote:Libration- which isn't real motion at all- is approximately (but not exactly) about the Moon's axis of rotation.
Libration is real relative motion. The Moon has a constant axial rotation rate, as we've both stated already. Libration is manifested as the illusion of the Moon rotating with a variable rate (and other lesser variabilities). But the illusion is caused by the real motion of (an observer on) the Earth and the Moon, relative to each other.Chris Peterson wrote:Libration is not real motion. It's an illusion created by our viewpoint, non-circular and tilted orbits. The Moon is rotating uniformly. An accelerometer on it wouldn't show any of the oscillating motion we call libration.Nitpicker wrote:Libration is real relative motion, resulting mainly from the variable orbital speed of the Moon and its constant axial rotation rate. Libration has three main components: longitudinal, latitudinal and diurnal. Longitudinal libration is largest component, and is about the Moon's axis of rotation. But the other components are quite significant, too, in proportion to the entire effect.Chris Peterson wrote:Libration- which isn't real motion at all- is approximately (but not exactly) about the Moon's axis of rotation.
Understood. But by far, the most common misconception I get in the classroom and at public outreach events is that libration is some kind of actual lunar motion. So I like to emphasize that it's an illusion, not any sort of physical motion. People already seem to have a hard time understanding that tidally locked bodies rotate at all. It just adds to the confusion when they try to picture them rocking- a confusion that has actually increased in recent years with the proliferation of movies showing that apparent rocking motion.Nitpicker wrote:Libration is real relative motion. The Moon has a constant axial rotation rate, as we've both stated already. Libration is manifested as the illusion of the Moon rotating with a variable rate (and other lesser variabilities). But the illusion is caused by the real motion of (an observer on) the Earth and the Moon, relative to each other.
Nitpicker wrote:I'll just describe it instead. It's very easy. In the Northern Hemisphere (only above the tropics for back safety), when the Moon is up and the sky is clear, go outside, stand up straight and face South. Now lean back until the top of your head points to Polaris. Hold that backwards lean. Now, relative to your lean, rotate your head left/right until you reach the Right Ascension of the Moon. Then tilt your head up/down until you reach the Declination of the Moon. If doing this for prolonged periods, you will need to slowly rotate your head to the right, in order to track the Moon accurately.Anthony Barreiro wrote:I hope you will change your avatar to illustrate your equatorially mounted head. I want to see how you've done that. I already know the angular width of your thumb at arm's length.Nitpicker wrote:... aligning our necks with the Earth's axis of rotation -- i.e. with an equatorially mounted head ... .
Yes, a lawn chair would be much more comfortable. As a matter of fact, a reclining lawn chair and a pair of binoculars on a clear warm night is pretty close to my idea of heaven. I don't do a polar alignment of my lawn chair, however. It's not comfortable to crane my neck around to the left or right. Rather, I point the chair toward the approximate azimuth of what I'm going to be looking at, and adjust the height of the seat back so my head will be supported at the right altitude. I once saw an article in Sky and Telescope magazine about a fellow who installed the motors and hand controller from a go-to telescope into a reclining easy chair. Now that would be sweet.BMAONE23 wrote:Of course it would be easier with the proper chaise lounge having the back reclined at the correct angle
Paddy O'Furniture says it's less stressful
I know a few people who've done that. But keep in mind such designs are always alt-az, so you'll still see the Moon rotating over the course of an evening. Unless you use it at one of the poles, of course. But that kind of eliminates the "warm summer evening" aspect.Anthony Barreiro wrote:I once saw an article in Sky and Telescope magazine about a fellow who installed the motors and hand controller from a go-to telescope into a reclining easy chair. Now that would be sweet.
Chris Peterson wrote:I know a few people who've done that. But keep in mind such designs are always alt-az, so you'll still see the Moon rotating over the course of an evening. Unless you use it at one of the poles, of course. But that kind of eliminates the "warm summer evening" aspect.Anthony Barreiro wrote:
I once saw an article in Sky and Telescope magazine about a fellow who installed the motors and hand controller from a go-to telescope into a reclining easy chair. Now that would be sweet.
http://www.washingtonpost.com/national/health-science/john-dobson-evangelist-for-amateur-astronomy-dies-at-98/2014/01/21/1901f2de-82a2-11e3-8099-9181471f7aaf_story.html wrote:
<<Dobsonian telescopes have made important contributions to astronomy, including the discovery in 1995 of Comet Hale-Bopp, the most-distant comet ever discovered by amateurs. One of its namesakes, Tom Bopp, was using a Dobsonian.
Alborzian, who had known Mr. Dobson since 1968, said he once urged Mr. Dobson to patent his design. He refused. “He said, ‘These are gifts to humanity,’ ” Alborzian recalled. “His goal was to open astronomy to the common man.”
In his 90s, Mr. Dobson was still a gypsy proselytizer for astronomy, lecturing and giving workshops as far away as Russia and China. Used to the most Spartan accommodations, he was known to sleep inside a telescope’s cylindrical tubes when he was on the road. “There’s one thing nice about sleeping in a telescope,” he told the Ventura County Star in 2007. “You can’t roll out of bed.”>>
You could bolt the chair on a platform tilted at 90 degrees minus your latitude, and use a harness to strap yourself in. An equatorial wedge lazy boy. Although that might be a lot of work simply to keep Mare Frigoris at the top of the Moon (or the bottom if you're down under).Chris Peterson wrote:I know a few people who've done that. But keep in mind such designs are always alt-az, so you'll still see the Moon rotating over the course of an evening. Unless you use it at one of the poles, of course. But that kind of eliminates the "warm summer evening" aspect.Anthony Barreiro wrote:I once saw an article in Sky and Telescope magazine about a fellow who installed the motors and hand controller from a go-to telescope into a reclining easy chair. Now that would be sweet.
Knock yourself out (but hopefully not literally!) Be sure to post a picture when you've got it working.Anthony Barreiro wrote:You could bolt the chair on a platform tilted at 90 degrees minus your latitude, and use a harness to strap yourself in. An equatorial wedge lazy boy.
