
Inconstant Moon: The Moon at Perigee and Apogee (1997) - nkurz
http://www.fourmilab.ch/earthview/moon_ap_per.html
======
autocorr
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_\(physics\))

~~~
taejo
> 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)

------
maxwelljoslyn
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!

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

------
draugadrotten
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/)

------
pmontra
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
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

      > 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"?

------
interfixus
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
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
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.

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

