Observing Project 5A Tycho L6 The Great Rayed Impact Crater

Tycho (L6) is one of the Moon's most beautiful signature craters. It sits in the far southern reaches of the Moon's visible face and is most famous for its characteristic "rays." Though no Apollo mission landed here, many at NASA called for a landing at this spectacular landmark. Such a landing never occurred because of the high risk inherent in landing on such rough terrain and because the trajectory required would preclude a free return trajectory. In 1968, the unmanned Surveyor 7 did land some 30 miles north of the crater rim in a bright ejecta blanket. Tycho is a relatively young crater, possibly less than 500 million years old. It is approximately 102 kilometers across and remarkably deep. The floor of the crater is approximately 4,800 meters below the rim. Tycho is the classic example of an

Figure 5.2. Tycho and its ray system. Image by author using Celestron C8 and a low-resolution video eyepiece.

impact crater. For many years it had been argued whether the Moon's craters were volcanic in nature or created by impacts. There seems very little doubt in the case of Tycho. As you look in on Tycho, notice that the crater floor is rough and it is bright. The material here is primarily anorthosite, the aluminum-rich material that makes up most of the lunar highlands. The same is true for the rays that emanate from the crater. That bright material is also anorthosite indicating that it is primarily surface material that had been blasted out of the impact site by whatever rocked the Moon's surface within the last 500 million years.

The walls of Tycho display the youthful nature of the crater. The walls are rough and terraced, suggesting that no eroding processes have taken place to age the walls. Also you should take note of the mountain formation on the crater floor. Tycho has a triple peaked mountain that rises from the crater floor, which towers nearly 5,000 feet above the crater floor. This feature is also indicative of an impact formation. A volcanic eruption would be unlikely to leave any peaks standing on the crater floor, never mind three of them. Can you bring enough magnification to bear on the crater to see all three? The triple peak should be easy in a telescope of 8 inches although I have also been able to see it clearly in scopes as small as 4 inches.

The best time to study Tycho, or any other lunar feature for that matter, is when the Sun is relatively low on the lunar horizon. Then the surrounding peaks and valleys create long shadows that cast the area in dramatic relief, showing just how rough the area in and around Tycho. If you study the area at local noon, the Sun casts very short shadows and thus even the roughest lunar surface features can appear flat. Local sunrise at Tycho occurs when the Moon is approximately eight days old. So look for Tycho when the Moon is about a day past first quarter. For the next forty-eight hours, the Sun is less than 20 degrees above the local horizon. Look again if you are a night owl when the Moon is about a day before last quarter (20 day old Moon) and Tycho is within about forty-eight hours of sunset.

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