Observing Project 5C Star Hopping Practice The Apollo 11 Landing Site

Selecting a landing site for the first lunar landing, mission planners for the flight were primarily interested in one thing, safety. The objective was to find a relatively flat spot with a paucity of craters or mountains that would allow for an approach to take place over a broad stretch of flat terrain. The logical choice would be an approach to the western side of a flat lava plain, since the Apollo missions orbited from east to west (retrograde). For the first landing, the planners chose the western side of the Sea of Tranquility. This particular spot also gave the advantage of a landing site near the lunar equator, which meant even as the Moon rotated underneath the orbit of the command module, the CM would not drift away from the landing site over several days. The low-latitude landing also made life much easier for the trajectory planners who were always adamant about maintaining the option for a free return in the event something went wrong.

Figure 5.4. Sea of

Tranquility. Can you find Apollo 11's landing site? NASA Lunar Orbiter IV image.

Figure 5.4. Sea of

Tranquility. Can you find Apollo 11's landing site? NASA Lunar Orbiter IV image.

In this project, we will attempt to zero in on the landing site of Apollo 11 using a technique that will become very valuable to you in later efforts to explore the deep sky. The technique is called "star-hopping." Star hopping involves beginning at a bright or distinct feature that is easily found, then from it, zeroing in on a fainter object by following a trail of other features. The features that we'll use are listed in the Lunar 100 and we'll travel from point to point in ascending order from easy to find to hard to find. You will not by any means be able to see the actual landing site on the surface but to identify the point of landing is exciting because it will build your confidence in your ability to navigate at the telescope. You will also have the gratification of looking at the spot where one of the most important events in the history of the humanity took place.

The mission of Apollo 11 was timed such that it would land at the target site just after local sunrise, about a day or so afterwards. This was done for the same reason that we like to observe lunar features when they are near the lunar terminator. The lunar shadows at that time would give the crew a good degree of depth perception to help them better judge their height and forward speed during the final moments of their manually controlled descent to the surface. With the Sun about ten degrees above the local horizon, shadow lengths are ideal. You will want to have about the same lighting conditions as the crew did. These conditions typically occur when the Moon is waxing and about five days old. You can also have success when the Moon is waning and sunset is nearing at Tranquility Base when the Moon is about twenty days old, but for the novice, this is far easier at sunrise.

Start out by locating the Sea of Tranquility (Figure 5.4). As the Moon waxes, two mare come into view near the Moon's east limb. Mare Crisium is the first to appear, then Mare Fecunditis appears to its southwest. Then over the next two days, sunrise occurs over Mare Tranquillitatis. Since the landing site is on the extreme western edge of the mare you must wait for the terminator to expose the entire area. Once you have identified the Sea of Tranquility, search along its southwestern edge for two twin craters in a northwest to southeast line nearly touching each other. Each of these two impact craters is approximately 30 km across. The northwest crater in this remarkable formation is called Ritter and its counterpart to the southeast is called Sabine. The two craters are listed together in the Lunar 100 as L38. With good shadow relief, these two craters should be easy to see with even low magnification. Now things get harder. Use the highest magnification eyepiece that the telescope and seeing conditions can bring to sharp focus and center on Sabine. The two craters are easily seen at the lower left in Figure 5.4 near the edge of the Sea of Tranquility. In the image, north is up and east is right. Remember to account for the orientation change in your telescope. If you are using a refractor or Cassegrain design with a star diagonal, north is up, but east is left. In a Newtonian, north is down.

From Sabine, look very carefully off towards the east. You are using very high magnification now because the next thing you are looking for is a string of three tiny craters each of which is less than three miles across. At high noon, you could never see them because without shadows to provide contrast they are just too small to be viewed. Shadow relief makes seeing them possible with steady air and effort. The westernmost crater in the string is called Aldrin and it is about 55 km east of Sabine. About another 49 km east of Aldrin is an even smaller crater called Collins and about 40km east of that crater is the largest of the string called Armstrong.

These three craters are named for the three astronauts who crewed the mission that landed just to the south of this spot and are collectively listed in the Lunar 100 as L90. The challenge is to find Collins. From Collins, scan directly south to a small bright crater called Moltke which is about 6 km across and some 88 km south of Collins near an outcropping of highlands. The crater is easily visible because of the contrast it makes with the surrounding basalt plains. If you imagine a line connecting Collins to Moltke, exactly halfway down this line, then about one-third of the length of the line to the west is the spot where Apollo 11 set down on July 20, 1969. If you cannot find Collins, then the landing site can also be found by drawing a line between Aldrin and Moltke. The site is on this line about one quarter of the distance from Moltke to Aldrin.

Later on you will apply this technique of using a bright object or an easy to find feature to navigate to more obscure objects and zero in on your treasure. Before computer driven telescopes flooded the market, star hopping was how amateurs learned their way around the sky and made even its faintest treasures as familiar as the features of any road map. Here we used the brightest features of the Moon to zero in on conspicuous features, then to find obscure ones on our way to one of the most historic places in the solar system. There are two important things also to remember when viewing the Moon and looking for fine details. First you should always use a neutral filter to reduce glare. A good Moon filter will cut the total light from the Moon by almost 90%. Secondly, find the highest power eyepiece that you can possibly use under the existing condition, put it in the scope and crank it up! There is an abundance of light available from the Moon to make crisp images possible where it otherwise would be a waste. So this is one of the places where when it comes to using high power, its time to go for it.

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