The most beautiful stage of a lunar eclipse is when the moon is almost totally eclipsed. This stage shows the various colors of the Earth's umbra well.

Introduction

A lunar eclipse happens when the moon passes through the shadow of the Earth, or in other words, when the sun, Earth and moon all lie on a straight line. The moon orbits the Earth once every 27.3 days, but since the plane in which the moon orbits the Earth is tipped w.r.t. the plane in which the Earth orbits the sun (about 5 degrees), we don't see a lunar eclipse every month, since not every month the three bodies align.

A lunar eclipse happens a few times per year on average (from no eclipses to 3 eclipses). Lunar eclipses happen more frequently than solar eclipses, because the Earth is larger than the moon, and because the angular extend of the sun is comparable to that of the moon, making solar eclipses very local. Lunar eclipses are less spectacular than solar eclipses, but the sight of a dark orange-brown moon between the stars is still very beautiful.

Viewing opportunities

Unlike solar eclipses, lunar eclipses are completely safe to watch. The full moon is very bright, but not bright enough to damage the eyes. Also, a lunar eclipse happens slowly, in about an hour (for total eclipse), compared to a total solar eclipse which typically only lasts a few minutes and is only visible on a very narrow strip across Earth. Lunar eclipses are visible to anyone on the night-side of Earth.

Since the moon passes through the shadow of Earth during an eclipse, the moon is always full when this happens. Also, because the moon is full, that means that a lunar eclipse is only visible at or after sunset. Since the eclipse phases last only a few hours, it is possible that a lunar eclipse is not visible for you, if it happens during your daytime.

The shadow of Earth at the location of the lunar orbit consists of a ring-shaped penumbra with the umbra at the center. The penumbral shadow is the location where Earth only partially blocks the sunlight from reaching the moon. The thickness of the penumbral shadow of Earth has the angular extend of the sun as seen from the Moon. Note that the Moon is approximately the same angular diameter as the penumbral ring, and thus the approximate angular diameter of the sun; this is utter coincidence and is the reason why solar eclipses last so short. A partially eclipsed Moon has been added to the diagram for comparison.

Shadow geometry

Earth's shadow (umbra) does not extend to infinity. Because the sun is so much larger than Earth, the shadow of Earth is shaped like a cone. The extend of that cone is approximately 1.4 million km (about 850,000 miles). For comparison, the lunar orbit is between 360,000 km and 400,000 km, roughly. Within this cone, the direct sunlight is completely blocked, and the only light in this umbral region is the reddish light due to the sunlight being refracted by Earth's atmosphere (by the same principle that makes a sunset red).

The cross-section of the shadow geometry, at the position of the moon, is drawn in the figure to the right.

Outside the umbral region, there is a region called the penumbral shadow (partial shadow), which is the region where Earth only partly blocks the sun; as seen from the moon the sun looks like a crescent here (like a partial solar eclipse as seen from Earth). The penumbral shadow is a consequence of the sun being an extended light source rather than a point light source. (By the same principle, you may have noticed that all the shadows cast by the sun here on Earth appear fuzzy, especially if the object casting the shadow is some distance from the place where you see the shadow. This is the same effect.)

The penumbral shadow region of Earth does extend to infinity, although at infinity the Earth looks so much smaller than the sun that the loss in light is negligible. The penumbral shadow merges gradually with the shadow-free region and the umbral shadow.

Phases of a lunar eclipse

The various phases of a lunar eclipse are depicted in the figure to the right. These phases are:

The various stages of a typical lunar eclipse, which times you can find tabulated for a particular eclipse.

• P1: the moon touches the penumbral ring of Earth's shadow. This stage is not visible
• U1: the moon touches the umbral part of Earth's shadow. This is when the partial eclipse starts. The moon now appears full, and the limb closest to the umbra appears a bit darker
• U2: the moon is completely eclipsed, and colors (usually) a dark orange brown
• U3: the end of the total eclipse. The second partial eclipse phase starts
• U4: the partial eclipse phase ends
• P2: the moon leaves the penumbral shadow of Earth

In case you were wondering, P stands for penumbral and U for umbral. Umbra and penumbra are Latin words for shadow and partial shadow.

There are thus three types of lunar eclipses: penumbral, partial, and total. A total lunar eclipse has all three eclipse types and is the most spectacular to watch. A partial eclipse is when the moon does not enter the umbral shadow completely (i.e. phase U2 and U3 don't occur). A penumbral eclipse is when the moon only crosses part of the penumbral shadow (i.e. none of the phases U1 through U4 occur). This type of eclipse usually goes unnoticed: almost no observers go out to watch such an eclipse, since it is little interesting.

A penumbral eclipse is hardly detectable. If you look carefully at the photo above, you will notice that the upper left limb of the moon appears slightly darker. The moon was about 75% into the penumbral shadow when the photo was taken.

Photography

There are various interesting things you can do for lunar eclipse photography. If you want nice pictures of the eclipsed moon, you will need a telescope mount or some other setup to guide the camera along the motion of the stars (or better, the moon), compensating for Earth rotation. You need this since typical exposures during the total eclipse can be up to a few minutes, and the moon and stars move appreciably during that time (the moon moves approximately 14.5 degrees per hour, and the stars 15 degrees per hour).

Also, you will need a long telephoto lens, a 500mm to 1000mm lens being a good choice. The longer the telephoto lens, the more need there is for the guide unit mentioned above.

As for film, I'd recommend you to use slide film, 800 or 1600 ISO for the total eclipse, and 100 ISO for the partial/penumbral eclipse phases. For the partial phases, you don't need a star (moon-) guiding unit. However, you will need to guide the camera if you use a long telephoto lens during the total eclipse. Also, the setup of the camera should be very rugged, or the exposures will be blurred due to wind or tremor from the camera shutter. It is best to use a black cardboard for the long exposures (> 1 second) and use that in combination with your camera's Bulb mode as a shutter in front of the lens.

The most difficult part of lunar eclipse photography is to determine the correct exposure during the total eclipse, since the moon can vary widely in brightness, depending on how much clouds and dust in Earth's atmosphere block the sunlight. The brightness of a total lunar eclipse is given by the so-called Danjon scale:

• L=0 moon almost invisible
• L=1 dark brown eclipse, lunar features very hard to see
• L=2 orange/brown color with dark central part of umbra; border of umbra may be brighter and blueish
• L=3 bright red/orange eclipse, with darker central part of umbra, very bright border
• L=4 very bright orange eclipse

Unless there has been a major volcanic eruption recently, expect L=2 to L=3 and adjust your camera settings accordingly beforehand. You can then change only if necessary. Keep in mind that if the moon dips deep into the umbral shadow, the moon will get much darker when it is near the center. Also, the shadow may not be as dark along every part of its border.

Now for the exposure guide: there is an extensive table below which lists the recommended exposure times for various phases of the moon and stages of a lunar eclipse. This guide is just that (a guide) and will get you the right results to within about one f-stop or so. You should definitely bracket your exposures (making several exposures differing in f-stop or shutter speed around the recommended value) for every exposure you deem important.

ISO	f-stop
100	2.8	4	5.6	8	11	16	22
200	4	5.6	8	11	16	22	32
400	5.6	8	11	16	22	32	44
800	8	11	16	22	32	44	64
1600	11	16	22	32	44	64	88
3200	16	22	32	44	64	88	128

Moon phase	Exposure time
Earthshine	1 min	2 min	4 min	8 min	15 min	30 min	60 min
Crescent	1/500	1/250	1/125	1/60	1/30	1/15	1/8
First/last quarter	1/1000	1/500	1/250	1/125	1/60	1/30	1/15
Gibbous	1/2000	1/1000	1/500	1/250	1/125	1/60	1/30
Full	1/4000	1/2000	1/1000	1/500	1/250	1/125	1/60
Partial eclipse - phase 0.50 - phase 0.30 - phase 0.20 - phase 0.10 - phase 0.05	1/2000 1/1000 1/500 1/250 1/125	1/1000 1/500 1/250 1/125 1/60	1/500 1/250 1/125 1/60 1/30	1/250 1/125 1/60 1/30 1/15	1/125 1/60 1/30 1/15 1/8	1/60 1/30 1/15 1/8 1/4	1/30 1/15 1/8 1/4 1/2
Total eclipse - L=4 - L=3 - L=2 - L=1 - L=0	1/2 2 8 30 2 min	1 4 15 1 min 4 min	2 8 30 2 min 8 min	4 15 1 min 4 min 15 min	8 30 2 min 8 min 30 min	15 1 min 4 min 15 min 1 hr	30 2 min 8 min 30 min 2 hr

[Exposure times for various phases of the moon and stages of a lunar eclipse. Times are in seconds unless specified. Part of this table is based on the data by Fred Espenak.]

A partial lunar eclipse, showing the sharp drop in brightness close to the umbral shadow.

How to use the table: choose a film speed, and aperture number (the aperture number is usually limited by your lens, if you are using a long telephoto lens). Then with that ISO and f-stop number, go straight down into the second table to the subject you want to photograph.

Note that, except for Earthshine, exposure times longer than about 4 minutes are really not practical, since you don't have unlimited amounts of time before the event ends. Also note that Earthshine and the various moon phases have nothing to do with lunar eclipse photography but occur in the table for completeness.

Some other photography I can recommend you to do:

• Photograph the stars and the eclipsed moon with a short telephoto lens, like 135mm.

• Do landscape-moon double exposures.

• Do a multi-exposure of the various eclipse phases on one photo, with the camera fixed on a tripod (like one exposure every 5 to 10 minutes with a 85mm lens). You use the Earth rotation to separate the several moons on the frame (the moon moves with about 14.5 degrees per hour).

• Do a multi-exposure with a star-guiding unit, to see the moon glide through the Earth shadow (which on the frame is fixed). You will need only about 5 exposures, more than one hour apart from eachother if you don't want the moons to overlap.

The partial eclipse is worth some additional notes. It is a misconception to think that you need to expose the photo according to the amount of surface area of the moon still sunlit. In the narrow penumbral zone the moon varies several stops in brightness, so you have to choose the exposure time for the part of the penumbral zone which you want properly exposed. The magnitude of the partial eclipse (the fraction of the diameter of the moon still outside the umbra) is listed in the table, along with the exposure time.

Out of paranoia, I tried a numerical simulation of the intensity of the penumbral shadow as a function of this magnitude, and plotted the intensity in f-stop scale versus the eclipse magnitude, or phase. Phase 0.0 corresponds to the place of the penumbral shadow closest to the umbra (i.e. the darkest part). Only a small crescent sun still illuminates the moon here. Phase 1.0 corresponds to the outer boundary of the penumbral ring, and is effectively the intensity of the regular full moon.

The drop in exposure stop as a function of eclipse magnitude. The drop in stop is with respect to a drop of 0 for the full moon (phase 1.0). Approximately halfway into the penumbral shadow ring, the stop is -1, thus an exposure time that is twice that required for the regular full moon.

The phase number is linear with the distance (radius) into the penumbral zone (I defined the phase number, or magnitude, as the fraction of lunar diameter still outside the umbra). From the graph, you can see that the place in the middle of the penumbral ring corresponds to a stop of -1, which means that you would have to expose a photo twice as long as the full moon. The data in the table above translate those drops in f-stop to exposure times, for convenience. The figure next to the graph shows a schematic close-up of the penumbral zone and the direction of increasing phase number.

Note that this data is only of limited use, since the moon will usually subtend a large part of the penumbral zone. Thus, the moon will vary in brightness appreciably (several stops) if it is in the penumbral zone. The data is useful, though, if you want to take long exposures of the moon being almost totally eclipsed, in order to photograph the bright limb and the dark brown umbra together.