Lunar Sunrise

The Moon doesn't have an atmosphere so you would expect the Sun to just appear there at sunrise without any warning, unlike twilight on Earth. Well here's a surprise...

The first thing you would like to do after having watched a total eclipse of the Sun is to press a Rewind button and see it all over again. It's really spectacular. Unfortunately, you normally have to wait at least a year or two for the next total eclipse to happen. And this might be over a remote stretch of ocean or the interior of Antarctica or some other place not on the list of your preferred budget airline.

But what would you say if I told you that you could watch a total eclipse of the Sun once every hour? That's no fairy tale, no witchcraft is involved and there are a few people who actually have had this experience. The trick is to fly to the Moon.

The Apollo spacecraft of some 40 years ago went into orbit around the Moon with a revolution period of two hours. So every two hours they could watch the Sun rise and set once. As a total eclipse of the Sun happens when the Moon totally blocks the Sun this amounts to two total eclipses each revolution.

Well, it rather amounts to half a solar eclipse. The great part about watching a total eclipse of the Sun from Earth is that the apparent diameter of the Moon is almost the same as that of the Sun and so the Sun's disks just gets covered completely. This enables you to enjoy a 360° panorama of the solar corona all around the Sun. If you watch a sunrise at the Moon the lunar surface will block out more than half the corona. So in what follows we should talk about lunar sunrise or sunset rather than an eclipse.

How then does a lunar sunrise look like? Let's hear some eyewitness reports, taken from the Air-to-Ground transcripts of the Apollo missions.

On 24 December 1968 Frank Borman, Jim Lovell and Bill Anders of Apollo 8 became the first humans to orbit the Moon. The following exchange between Lovell and Houston happened at 19:11 GMT, 24 December 1968:

LOVELL:
Houston, this is Apollo 8. We have a little piece of useful information if you're interested in deliberating over it.
HOUSTON:
Go ahead. Say again.
LOVELL:
Roger. Our first control point is very near the terminator, and as the optics were tracking it, I had occasion to watch the sun come up. And at about 2 minutes before sunrise, you get - the limb begins to brighten up into sort of a fine white haze, a fine glow completely over the space just behind the limb.
HOUSTON:
Roger. I understand. About 2 minutes before the sun comes up, you get a fine white haze radiating out from behind the limb. How far out does it extend?
LOVELL:
It goes up quite a ways. It takes a fan shape, unlike the sunrise on earth where the atmosphere affects it. This just sort of is a complete haze all over the local area. It's concentrated at the exact spot where the sun comes up at ignition and then goes away from the sun spots. Very interesting.

As the Apollo spacraft took 120 minutes to orbit the Moon it would cover three degrees per minute. So Lovell would have seen some hazy light when the Sun was still 6° below the horizon. Although coronal structures can extend that far out what Jim Lovell saw at that time most likely was the zodiacal light. This is sunlight being scattered by dust particles lying in the plane of the ecliptic and can also be observed at dark places on Earth as a faint cone appearing well before sunrise until the twilight gets too bright. Detailed coronal structures were probably visible for the final 15 seconds or so.

Judging by this exchange the Apollo 8 astronauts seem to have been unprepared for this effect. That's not surprising as this mission had been put together quickly and the astronauts had more important things to worry about (like getting safely to the Moon and back again). So even if someone had thought about this before it would not have been on the priority list of crew training.

But now astronauts had become aware that there is something special to see at lunar sunrises and sunsets. Shortly after Apollo 11 had gone into orbit around the Moon the astronauts made the following remarks at 12:30 GMT, 19 July 1969.

COLLINS:
Roger. What sort of settings could you recommend for the solar corona? We've got the Sun right behind the edge of the Moon now.
ALDRIN:
It's quite an erie sight. There is a very marked three-dimensional aspect of having the Sun's corona coming from behind the Moon the way it is.
ALDRIN:
And it looks as though - I guess what's giving it that three-dimensional effect is the earthshine. I can see Tycho fairly clearly - at least if I'm right side-up, I believe it's Tycho, in moonshine - I mean, in earthshine. And, of course, I can see the sky is lit all the way around the Moon, even on the limb of it where there's no earthshine or sunshine.
HOUSTON:
Roger. If you'd like to take some pictures, we recommend using magazine Uniform which is loaded with high speed black and white film, interior lights off, electric Hasselblad with the 80-millimeter lens. And you're going to have to hand-hold us, I guess. We're recommending an f-stop of 2.8, and we'd like to get a sequence of time exposures. Over.
ARMSTRONG:
And Houston, I'd suggest that along the ecliptic line we can see the corona light out to two lunar diameters from this location. The bright light only extends out about an eighth to a quarter of the lunar radius.

It is interesting that the crew of Apollo 11 found time for these observations as they sure must have been concerned with the upcoming lunar landing!

The final Apollo missions were completely dedicated to science. While John Young and Charlie Duke of Apollo 16 were on the lunar surface Ken Mattingly circled the Moon alone in the Command Module and was kept busy by scientific observations from orbit. Here's what he had to say about lunar sunrise on 21 April 1972:

MATTINGLY:
I saw one thing that was - that was new about the zodiacal-light world. You don't realize that that stuff is actually reasonably bright until - you get yourself well dark-adopted then you sit up there long enough to recognize it. But almost - almost immediately after - oh, I'd say within - certainly within 5 minutes and maybe a little sooner after LOS, there was enough zodiacal light or solar corona - whichever is the proper technical name - that you could see a distinct horizon. And it just got more distinct right up until sunrise. I never did see any prominences or anything like that. I kind of thought that maybe we would, but I guess those are too small a scale. But, just prior to sunrise, you see a lot of long streaks and that's - well, I say just prior - that's like just a few seconds before sunrise. All of a sudden, you get these very prominent rays coming out and then the Sun's right there. You don't want to be looking at that.

This is a very detailed description indeed. Anyone who has ever seen a total eclipse of the Sun will recognize what Mattingly is talking about. So a lunar sunrise goes like this: You first see the faint cone of the zodiacal light similar to what you would see at a dark place on Earth. But while on Earth the whole sky gets brighter with twilight it's the zodiacal light which gets brighter the closer the Sun gets to the horizon. Finally bright coronal streamers show up and it gets brighter still until the Sun's limb appears at the horizon.

40 years ago all the attention was on Earth rise pictures and I cannot remember any press reports about how a sunrise looks at the Moon. Of course seeing the Earth rise above a barren lunar landscape was something completely new and inspiring and people probably felt they had seen plenty of sunrises on Earth anyway. But judging by the reports of the Apollo astronauts and by my own experience of having seen a number of total eclipses of the Sun lunar sunrises are as spectacular to watch!

How does a lunar sunrise look like if you are at the surface of the Moon rather than just seeing it from orbit? This will depend on where you are. If you are not too far from the (lunar) equator at about the places where the Apollo landings happened it should look similar to what was reported from orbit - only that it happens a lot slower because the Moon takes a whole month to turn around while our orbit lasted just two hours. You probably see the zodiacal light at least a day (or even several days if you are well dark adapted) before sunrise and bright coronal streamers might be visible during the final hour or two.

But the first permanent human base on the Moon might not be anywhere near the equator. Current thinking at NASA seems to favour a base near the lunar south pole. This is a very strange place indeed. There might be certain mountain peaks where the Sun never sets which would mean a continuous supply of solar energy. There are also some craters which are always in shadow and might contain some ice deposits and thus a ready supply of water. But all that still needs to be confirmed.

Most places near the lunar south pole will see the Sun some of the time. The rotational axis of the Moon is inclined to the ecliptic at 1½° (as compared to 23½° for the Earth). This creates a small seasonal effect. Similar to what happens on the Earth's poles the Sun will wind its way up and stay above the horizon for the summer (but only at an elevation of 1½°) and then disappear for the winter over the course of a year.

Let's assume we are close to the lunar south pole and the Sun is just below the horizon. Would the view be similar to what you can see at sunrise near the lunar equator? Not at all! The reason for this is that the part of the Sun just about to rise is the solar south pole. The first casualty will be the zodiacal light as this extends along the ecliptic which is perpendicular to the polar axis of the Sun and thus will be missing over the poles. You might still be able to see some of the zodiacal light but rather than extending vertically upwards it will go to the sides and show up along the horizon.

As to the corona what can be seen will depend on the solar cycle. The Sun's activity varies (roughly) with an 11-year cycle and so does the corona. Compare the picture of the total eclipse from November 1994 which happened close to solar minimum with the one from July 2001 close to solar maximum. While during solar maximum the bright coronal streamers go in all directions solar minimum is characterized by short brushlike polar plumes while the long bright streamers are concentrated along the equator and thus would be missing at the lunar south pole at those times.

Because the Sun will always be close to the horizon near the lunar south pole you might be able to see a few sunrises and sunsets on a single lunar day (one month) if you are at a place which is surrounded by low hills and valleys which will cover and uncover the Sun while the Moon turns around.

Buzz Aldrin mentioned a three-dimensional effect due to the illumination of the lunar landscape by earthshine. This will depend on the phase of the Earth which is opposite to that of the Moon. For a sunrise close to the eastern limb of the Moon (close to Mare Crisium) the Earth would be almost fully illuminated and light up the lunar landscape. For a sunrise close to the western limb the Earth would just be a small crescent and the lunar landscape would remain almost dark. It's the other way around for sunsets.

Once again the polar regions have it their own way. An effect called libration (which is largely due to the motion of the Moon along its inclined elliptic orbit around the Earth) causes the position of the Earth as viewed from the Moon to vary by a few degrees over the course of a month. So sometimes the Earth will be just above the horizon over the poles and sometimes just below. And what phase the Earth has while being over the horizon varies. So any combination seems to be possible there.
 
Have you decided on your favorite lunar destination yet? Will it be one of the nice hotels near the equator or are you attracted by the moody south pole? As to me, I've made up my mind after checking my bank account - if anything at all it will be earthbound holidays...