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# Thread: How fast does the moon move through the sky

1. Often people fudge(d) a night time landscape including a moon with double exposure, and these days more obvious and easier trickery

The moon moves fast enough to see an obvious movement when you shoot a moon lit scene with LF at f22, exposure times tend to be in the order of a minute or longer. But you should be OK with Tmax 100 and a fast lens with 35mm working around f4/f5.6

Ideally you need to shoot at dusk before it gets totally dark so taht the exposures balance for the moon & the scene, it's a very short threshold 10 mins maybe here in the UK but less further south maybe only 2 mins where we live in Turkey.

The secret with night shots & the moon is the twi-light zone

Ian

I am looking at an image taken via my cropped sensor digital with 200mm lens. On this frame, the moon occupies 1/13th of diagonal distance. This is equivalent of using 300mm lens on full frame camera. If I used 85mm lens, the size will be 1/3.5 of that. That would mean it will be 1/45 of the diagonal distance. If I printed this on 8x12 paper (to keep the aspect ratio intact and it has 14.5 inch diagonal distance), the moon will be about 1/3" in diameter.

My exposure was 1/500th at f/5.6, ISO=200.

Good luck.

3. The moon moves 360 degrees eastward against the background stars in about 27.3 days. So that's about 13 degrees per day. The earth also spins at 15 degrees per hour, much more rapidly than the moon's apparent progress against the stars. That's the motion that you're really concerned with.

After the calculations are done, the short answer is this:

The longest exposure in seconds that you can use to freeze the moon with no apparent motion blur on the film is 250/focal length of the lens.

See Astrophotography for the Amateur, 2nd. ed., Michael A. Covington.

This of course, assumes that you're using a fixed tripod. A driven camera or telescope mount will allow longer exposures.

The diameter of the moon subtends about 0.5 degrees of arc. That's small.

Lee

4. Originally Posted by Ian Grant
Often people fudge(d) a night time landscape including a moon with double exposure, and these days more obvious and easier trickery

The moon moves fast enough to see an obvious movement when you shoot a moon lit scene with LF at f22, exposure times tend to be in the order of a minute or longer. But you should be OK with Tmax 100 and a fast lens with 35mm working around f4/f5.6

Ideally you need to shoot at dusk before it gets totally dark so taht the exposures balance for the moon & the scene, it's a very short threshold 10 mins maybe here in the UK but less further south maybe only 2 mins where we live in Turkey.

The secret with night shots & the moon is the twi-light zone

Ian
That's interesting Ian. On TMX I will need 20 seconds (including reciprocity adjustment) at f5.6 (can't open up wider or won't have sufficient depth of field). And that's really the minimum exposure I can get away with, since that will place most of the picture area and subject matter on zones III-IV.

Ideally I'd be able to do the whole thing at twilight but in this case I cannot. It's one of those scenes that strikes you out of nowhere as you drive by. I noticed it by accident but the moon has to be in just the right place within the picture, and in fact the moon is hidden behind other subject matter until it rises to just that spot. All this currently occurs at around 10:30pm, well after dark.

Anyhow I wonder if 20 seconds will cause blur, or if I should just go with a faster film. With TMY2 I could probably get it down to 4-5 seconds.

5. Originally Posted by Michael R 1974

I guess ideally I'd make a first exposure for the scene before the moon is in frame, and then just add a short second exposure when the moon is in place. But I can never rely on double exposures with a roll film camera. Too bad I won't be able to use sheet film for this one.
I suppose you can leave your shutter open, and use a cardboard in front of the lens as shutter. This way you don't have to recock the shutter and don't have alignment problems with the two exposures.

Do a first exposure by opening 20 seconds, then cover the lens with the cardboard, cloth, whatever, then re-expose for the moon.

I see this as something that will need several attempts before getting it right. The moon is much brighter than the surrounding area you are going to include in the composition (I infer this from your stated 20" exposure at 100 ISO). I suppose you can go down to 1/4" with the cardboard method.

The full moon is around EV 14 or EV 15 at ISO 100. That means 1/8 or 1/4 at f/64. You could apply the cap to a ND filter, and screw it to the lens with the cap on, then put the cardboard in front of the lens, and take the cap away, use the cardboard as a "shutter" with a rapid hand gesture (1/4" or so).

So the sequence would be:
Open shutter with cardboard over lens (no necessity of mirror lock-up);
Take cardboard away for 20 seconds or desired exposure, without moon;
Put cardboard in front of lens;
While keeping the cardboard in front of the lens, screw ND filter with cap on it;
Wait for the moon to rise and to go in position;
While keeping the cardboard in front of the lens, take the lens cap away;
Do a short exposure for moon details using the cardboard, that's something like 1/4 or 1/8 and is not going to produce a blurred moon;
Close shutter.

Repeat until result is satisfying.

As an alternative: just make two exposures and sandwich them while printing, the moon exposure will have density only on the small part occupied by the moon, no disturb to the first exposure.

Fabrizio

PS A camera with a mechanical B shutter is necessary because you'll have to wait for hours with the shutter opened.

6. Originally Posted by Lee L
The moon moves 360 degrees eastward against the background stars in about 27.3 days. So that's about 13 degrees per day. The earth also spins at 15 degrees per hour, much more rapidly than the moon's apparent progress against the stars. That's the motion that you're really concerned with.

After the calculations are done, the short answer is this:

The shortest exposure in seconds that you can use to freeze the moon with no apparent motion blur on the film is 250/focal length of the lens.

See Astrophotography for the Amateur, 2nd. ed., Michael A. Covington.

This of course, assumes that you're using a fixed tripod. A driven camera or telescope mount will allow longer exposures.

The diameter of the moon subtends about 0.5 degrees of arc. That's small.

Lee
Yikes that would mean 250/85 so we're talking 3 seconds. Wow that is not much at all.

7. Originally Posted by Michael R 1974
Yikes that would mean 250/85 so we're talking 3 seconds. Wow that is not much at all.
This is why you need to shoot at dusk, in that twilight zone, effectively depending on the exposure for the rest of the scene you can make the image appear to have been taken at night, or in daylight, anyway if the ambient lights too low the moon will be very over exposed anyway.

It's easier to do than explain. Think of the famous AA shot that's twilight.

Ian

8. Originally Posted by tkamiya
With 85mm lens, the moon will basically appear as a dot. There won't be much detail to speak of at all. Also, moon is very bright. Depending on your aperture setting, it will bloom quite a bit further obscuring details.

According to the Wiki page, it moves just about its diameter per hour. That means if you kept the shutter open for a whole minute, it will move 60th of its diameter.

How large are you going to print this?
Something wrong there -- I have photographed lunar eclipses, taking one image every 10 minutes -- and the moon has moved several diameters in that time. Plus it would never get across the night sky at that rate. I do not know how many diameters it is across the sky, but it is far more than the number of hours of darkness.

9. Originally Posted by Vaughn
Something wrong there -- I have photographed lunar eclipses, taking one image every 10 minutes -- and the moon has moved several diameters in that time.
tkamiya is quoting the wikipedia description of the motion against the background stars, caused by the moon's orbital motion about the earth, and not considering other motions. What caused nearly all of the offset in your series is the rotation of the earth, not the moon moving in its orbit. Two separate causes of apparent motion being confused/conflated/ignored with false conclusions on tkamiya's part.

Lee

And so we don't dwell on our own misunderstandings, here's some bad astronomy straight from Harvard: http://www.youtube.com/watch?v=p0wk4qG2mIg
None of the interviewed folks appear to have been from the southern hemisphere.

10. I think I am contradicting myself here, too. If I said earth's rotation is going to be more of a factor, that would mean 360/24 = 15 degrees. The moon certainly doesn't occupy 15 degrees of arc.

Please ignore everything I said. I'm going back to my cave and punish myself.

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