Please help me find when the moon disappears, so I can watch it.
October 23, 2009 10:27 PM   Subscribe

Back about a decade ago, I happened to be watching in early evening as the moon went from thin sliver to complete disappearance. It was one of those unexpected and somewhat wondrous moments of my life (due to many circumstances of the time, and the unplanned coolness of it). I'd like to see it again. Is there some schedule for me to tell when this is going to happen again visible from my area?

53N,-122.36W, if that helps.
posted by Kickstart70 to Science & Nature (8 answers total) 6 users marked this as a favorite
NASA give us the lunar eclipse schedule:
posted by Jon_Evil at 10:34 PM on October 23, 2009

That wasn't an eclipse.
posted by floam at 10:35 PM on October 23, 2009

I take you're talking about just happening to witness the normal lunar cycle as it slips into a full new moon. This is about as close as I can get you tonight.
posted by 517 at 10:43 PM on October 23, 2009 [1 favorite]

It was probably setting, rather than going from very old moon to a new moon.
And if it was early evening, it meant that you were seeing the moon after the sun
had set, which meant that it would be waxing, rather than waning, which
again seems to indicate that it was setting. It could also have been obscured
a cloud or mist. The apparent proximity of a new moon to apparent position of
the sun, and the glare from that adjacency makes observation of a "newish moon"
very difficult.

Now, if you were watching a lunar eclipse, it would have looked a little different
from a normal phase of the moon, and you could have seen the moon "go out".
You could have indeed seen this at sunset, opposite of the sun position.

You might make a point of watching the newest moon, soon after sunset, to see
if you saw the same thing, and are perhaps misremembering it. You might also
make a point of watching lunar eclipses.
posted by the Real Dan at 10:48 PM on October 23, 2009

I agree that you probably saw the last waning sliver slip behind a cloud. The movement from that point to true new moon is much more gradual.

Most calendars will tell you when full moon and new moon occur.
posted by megatherium at 5:08 AM on October 24, 2009

Nope, I did not watch the last waning sliver slip behind a cloud. Also, not an eclipse. I watched the last waning sliver as it "waned" completely.

Thanks, 517, I'll try to watch at "NOV. 16 19 14"(-8 hours for my timezone) and hope for good weather.
posted by Kickstart70 at 8:09 PM on October 24, 2009

Interesting question! If it was a crescent moon, near the sun in the early evening, it couldn't have been an eclipse; eclipses happen when the moon is at opposition, and it's full. (You can see a lunar eclipse in the early evening, since lunar eclipses are visible from half the globe and it's always early evening somewhere. But a full moon in early evening is in the east.)

The "instant" of the new moon, which is what's tabulated by the Naval Observatory (already linked by 517), is the instant when the angle between the sun and the moon is the smallest for a given month. That minimum angle is not the same every month, though, because the plane of the earth-moon orbit is tilted relative to the ecliptic. Some months the moon gets no closer than 5° to the sun, which is about ten moon diameters; some months the moon passes directly in front of the sun, which is a solar eclipse. This minimum angle also depends on where you are (otherwise you could see solar eclipses everywhere); I think the variation in the closest approach depending on your location is about one or two moon diameters, i.e. up to 1°.

So one unknown is, how close does the moon have to be to the sun for its edge to "wink out"? If it is, say, 7°, then this will be visible somewhere on the earth every new moon. If the winkout distance is, say, 2°, only some new moons will wink out, and then only from some observing locations. Observing location is another unknown, since a moon 5° from the sun sets only 20 minutes after and is viisble in the twilight for much less than an hour.

OH GOD DAMMIT, an early evening moon is WAXING (just like the Real Dan wrote yesterday), so it's getting bigger, not smaller. If you saw this moon in the early evening it was not waning. The waning moon would disappear just before dawn --- but it does that anyway, because the sun comes up. The waxing moon might suddenly appear as it gets far enough away from the sun --- but that's what happens as evening moves into twilight. So what you are describing is basically impossible to see. It was probably some sort of faint cloud.

I define eponysterical.
posted by fantabulous timewaster at 10:41 PM on October 24, 2009 [1 favorite]

Y'know...I remember the faded light, perhaps it WAS morning, on my walk to work. I'm going to have to think on this further.
posted by Kickstart70 at 11:12 AM on October 28, 2009

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