5 ms·

Inconstant Moon: The Moon at Perigee and Apogee (1997)

81 points by nkurz 10y ago

rowanseymour 10y ago
Eccentric orbits never really made sense to me until I started playing Kerbal Space Program.
interfixus 10y ago
I remember stumbling across fourmilab twenty years ago, and being instantly bowled over: So this is what a real website looks like. The breath and depth and variety of its resources - all public domain, btw. - were astounding then, and have, amazingly, kept to that standard ever since. My favorite go-to site for two decades running.
- lolc 10y ago
  I remember reading about the "gravitaiton experiment" for your basement and now I'm pleasantly surprised to see the page was updated: "Bending Spacetime in the basement" https://www.fourmilab.ch/gravitation/foobar/ https://www.fourmilab.ch/gravitation/foobar/
  - david-given 10y ago
    I keep getting surprised about how easy gravity is to demonstrate on the human scale. I should find a quiet room and actually do that experiment. Just because. Tangentially related fact: in flat space, a marble will orbit a bowling ball at a distance of about half a metre, with a period of about a day.
- _delirium 10y ago
  The various projects and hobbies that go into that site are basically John Walker's full-time job for the past ~20 years as I understand it, since he retired from AutoDesk with a few tens of millions.
autocorr 10y ago
This is a neat article, but I'd just like to point a common misconception that's made in the "How bright is the moonlight?" section. For a field that studies light, astronomers and physicists have made quite a few words to describe how "bright" something is: luminosity, flux, intensity, and brightness [1]. Somewhat confusingly, all of these words have different meanings, and also have infrequently used synonyms, like irradiance. In this case, the article makes the cardinal sin of physicists which is to confuse brightness and intensity with flux. If you put a piece of film on the ground on different nights for perigee and apogee, it would be more exposed on the night the moon was closer (perigee) and less exposed on the night it was further away (apogee). This is because the moons flux at the Earth's surface is lower according to the inverse square law. However, if you have a camera that can resolve the Moon across many pixels or bits of film, you will find that it is exposed to the same level! This is because the intensity and brightness are the same, and this is ultimately due to the fact that the Moon's distance from the Sun stays approximately constant. This is counter-intuitive (and something I've asked intro astronomy classes). Think of it this way, if you put the Moon twice the distance from the Earth, its flux will decrease by a factor of four. But, you will also put all of that flux into a smaller patch of the sky: twice the distance for a circular Moon means the area of the Moon will now by four times smaller. These precisely cancel each other, and the "light density" stays the same. Now if your eye could add up all light falling on your retina (flux), you could tell a difference, but since what you will perceive is the intensity/brightness, they will look the same. [1] See the comical number of units in the table on Wikipedia https://en.wikipedia.org/wiki/Intensity_(physics) https://en.wikipedia.org/wiki/Intensity_(physics)
- deleted 10y ago
  [deleted]
- maxwelljoslyn 10y ago
  Thanks for this informative reply. Today I learned something.
- taejo 10y ago
  > These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part of a scene? You can walk right up to that thing, use your light meter (either the one in your camera or a handheld) to set the exposure, and then walk away, recompose, change your lens... and still get the correct exposure. Until I understood this, I didn't really "get" how a hand-held light meter could be useful. There are two caveats to this. Make sure you don't cast a shadow over the subject when you're metering it; and if the subject is emitting light or has specular reflections, then it might not emit equally in all directions. On this latter point, an interesting question is why the moon appears as a disk of constant brightness, but the sun is darker around the edges (BUT DON'T LOOK AT THE SUN WITHOUT PROPER PROTECTIVE EQUIPMENT, obviously)
draugadrotten 10y ago
I was struck by the paragraphs about using film and transferring it using photoCD to get consistent lighting and scale. Oh my, digital photography really have changed things. For modern perigree chasers, I can very much recommend PHOTO PILLS, http://www.photopills.com/ http://www.photopills.com/ The app is used by many of my favourite photographers including myself :) and Chris Burkard http://www.chrisburkard.com/ http://www.chrisburkard.com/
maxwelljoslyn 10y ago
From the article, in the section "A Sense of Scale": "On [the scale of the above image], all human spaceflight with the exception of the Apollo lunar missions has been confined to a region of two pixels surrounding the Earth; seeing the Moon's orbit in its true scale brings home how extraordinary an undertaking the Apollo project was. Of all the human beings who have lived on Earth since the origin of our species, only 24 have ventured outside that thin shell surrounding our Home Planet. Even the orbit which geosynchronous communications satellites occupy is only a little more than a tenth of the way to the Moon." Amazing!
pmontra 10y ago
Larry Niven's Inconstant Moon from the first line of the article https://en.m.wikipedia.org/wiki/Inconstant_Moon https://en.m.wikipedia.org/wiki/Inconstant_Moon I read it many years ago, maybe in another collection.
- pavel_lishin 10y ago
  I recall him mentioning that he would have liked to see it as a television series, or someone approached him about making it one. I think now would be a perfect time to do so - I'd love to see a Netflix or an HBO take on something like this.
falsedan 10y ago
> In addition, the tidal effect of the Sun's gravitational field increases the eccentricity when the > orbit's major axis is aligned with the Sun-Earth vector or, in other words, the Moon is full or new. I don't understand this without a diagram: isn't the major axis of the elliptical orbit independent of the Moon's phase? Or is this an over-abbreviation of the statement, "the effect is most pronounced when the major axis of the orbit is aligned to the Sun-Earth vector, and coincides with a full or new moon"?
tgb 10y ago
I've always loved this gif from Wikipedia: https://commons.m.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007_450px.gif#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Lunar_libration_wi...
deleted 10y ago
[deleted]

81 points by nkurz 10y ago

Eccentric orbits never really made sense to me until I started playing Kerbal Space Program.

I remember stumbling across fourmilab twenty years ago, and being instantly bowled over: So this is what a real website looks like. The breath and depth and variety of its resources - all public domain, btw. - were astounding then, and have, amazingly, kept to that standard ever since. My favorite go-to site for two decades running.

lolc 10y ago
I remember reading about the "gravitaiton experiment" for your basement and now I'm pleasantly surprised to see the page was updated: "Bending Spacetime in the basement" https://www.fourmilab.ch/gravitation/foobar/ https://www.fourmilab.ch/gravitation/foobar/
- david-given 10y ago
  I keep getting surprised about how easy gravity is to demonstrate on the human scale. I should find a quiet room and actually do that experiment. Just because. Tangentially related fact: in flat space, a marble will orbit a bowling ball at a distance of about half a metre, with a period of about a day.
_delirium 10y ago
The various projects and hobbies that go into that site are basically John Walker's full-time job for the past ~20 years as I understand it, since he retired from AutoDesk with a few tens of millions.

autocorr 10y ago

This is a neat article, but I'd just like to point a common misconception that's made in the "How bright is the moonlight?" section. For a field that studies light, astronomers and physicists have made quite a few words to describe how "bright" something is: luminosity, flux, intensity, and brightness [1]. Somewhat confusingly, all of these words have different meanings, and also have infrequently used synonyms, like irradiance. In this case, the article makes the cardinal sin of physicists which is to confuse brightness and intensity with flux. If you put a piece of film on the ground on different nights for perigee and apogee, it would be more exposed on the night the moon was closer (perigee) and less exposed on the night it was further away (apogee). This is because the moons flux at the Earth's surface is lower according to the inverse square law. However, if you have a camera that can resolve the Moon across many pixels or bits of film, you will find that it is exposed to the same level! This is because the intensity and brightness are the same, and this is ultimately due to the fact that the Moon's distance from the Sun stays approximately constant. This is counter-intuitive (and something I've asked intro astronomy classes). Think of it this way, if you put the Moon twice the distance from the Earth, its flux will decrease by a factor of four. But, you will also put all of that flux into a smaller patch of the sky: twice the distance for a circular Moon means the area of the Moon will now by four times smaller. These precisely cancel each other, and the "light density" stays the same. Now if your eye could add up all light falling on your retina (flux), you could tell a difference, but since what you will perceive is the intensity/brightness, they will look the same. [1] See the comical number of units in the table on Wikipedia https://en.wikipedia.org/wiki/Intensity_(physics) https://en.wikipedia.org/wiki/Intensity_(physics)

deleted 10y ago
[deleted]
maxwelljoslyn 10y ago
Thanks for this informative reply. Today I learned something.
taejo 10y ago
> These precisely cancel each other, and the "light density" stays the same. This is very useful for a photographer! Want to correctly expose some small part of a scene? You can walk right up to that thing, use your light meter (either the one in your camera or a handheld) to set the exposure, and then walk away, recompose, change your lens... and still get the correct exposure. Until I understood this, I didn't really "get" how a hand-held light meter could be useful. There are two caveats to this. Make sure you don't cast a shadow over the subject when you're metering it; and if the subject is emitting light or has specular reflections, then it might not emit equally in all directions. On this latter point, an interesting question is why the moon appears as a disk of constant brightness, but the sun is darker around the edges (BUT DON'T LOOK AT THE SUN WITHOUT PROPER PROTECTIVE EQUIPMENT, obviously)

draugadrotten 10y ago

I was struck by the paragraphs about using film and transferring it using photoCD to get consistent lighting and scale. Oh my, digital photography really have changed things. For modern perigree chasers, I can very much recommend PHOTO PILLS, http://www.photopills.com/ http://www.photopills.com/ The app is used by many of my favourite photographers including myself :) and Chris Burkard http://www.chrisburkard.com/ http://www.chrisburkard.com/

maxwelljoslyn 10y ago

From the article, in the section "A Sense of Scale": "On [the scale of the above image], all human spaceflight with the exception of the Apollo lunar missions has been confined to a region of two pixels surrounding the Earth; seeing the Moon's orbit in its true scale brings home how extraordinary an undertaking the Apollo project was. Of all the human beings who have lived on Earth since the origin of our species, only 24 have ventured outside that thin shell surrounding our Home Planet. Even the orbit which geosynchronous communications satellites occupy is only a little more than a tenth of the way to the Moon." Amazing!

pmontra 10y ago

Larry Niven's Inconstant Moon from the first line of the article https://en.m.wikipedia.org/wiki/Inconstant_Moon https://en.m.wikipedia.org/wiki/Inconstant_Moon I read it many years ago, maybe in another collection.

pavel_lishin 10y ago
I recall him mentioning that he would have liked to see it as a television series, or someone approached him about making it one. I think now would be a perfect time to do so - I'd love to see a Netflix or an HBO take on something like this.

falsedan 10y ago

> In addition, the tidal effect of the Sun's gravitational field increases the eccentricity when the > orbit's major axis is aligned with the Sun-Earth vector or, in other words, the Moon is full or new. I don't understand this without a diagram: isn't the major axis of the elliptical orbit independent of the Moon's phase? Or is this an over-abbreviation of the statement, "the effect is most pronounced when the major axis of the orbit is aligned to the Sun-Earth vector, and coincides with a full or new moon"?

tgb 10y ago

I've always loved this gif from Wikipedia: https://commons.m.wikimedia.org/wiki/File:Lunar_libration_with_phase_Oct_2007_450px.gif#mw-jump-to-license https://commons.m.wikimedia.org/wiki/File:Lunar_libration_wi...

deleted 10y ago

[deleted]