In this lab you will make observations of a few specific features on the Moon, in particular using the astrometric eyepiece to measure the sizes of the objects. In the first Moon Lab you used the method of transit timing to measure the angular and actual sizes of a few craters on the Moon. The astrometric eyepiece simplifies this process greatly since one can read the size of an object almost directly off the linear diameter scale. Recall that each tic mark on the linear diameter scale equals 21 arc seconds (21”), so the angular size on any object is
Angular size (in ”) = (# of tics) • 21”/tic.
and the actual size of an object on the Moon (in km) is
Size(in km) = size (in arc seconds) • 1.86° ÷ cos(long)
where cos(long) is the cosine of the object’s longitude on the Moon (which can be found from the lunar map). For objects within 10° of 0° longitude, the correction for longitude is small enough one can leave it off and just multiply the angular size by 1.86. Note that this formula assumes one has measured the angular size of the object across its east-west dimension. Thus, apart from the exceptions noted, you must have the linear scale of the astrometric eyepiece oriented east-to-west.
In addition to measuring the sizes of a few features on the Moon, you will make written observations of the features’ appearances. Here are the features you will observe:
Objects near 0° longitude:
Montes Apenninus (Apennine Mountains – for this one orient your linear diameter scale along the length of the mountains.)
Crater Plato
Plus pick any one of these craters: Archimedes, Ptolemaeus, Eratosthenes
Objects away from 0° longitude:
Mare Serenitatis
Mare Crisium
Plus pick any one of these craters: Copernicus, Delambre, Langrenus, Reinhold
Materials
Telescope with astrometric eyepiece and moon filter, lunar map, calculator
Procedure
· Set up your telescope and locate the Moon in your astrometric eyepiece. You will probably want to find the Moon and locate each object using your low-power eyepiece first. You will also want to install the moon filter in your astrometric eyepiece to cut down on the Moon’s glare.
· Make note of the time, equipment and conditions.
· Locate and identify the first object on your list.
· Record your observations of its appearance. Note its shading—is it light or dark. Briefly describe its general appearance. For craters, make note whether the floor of the crater is smooth or rough, if it has a central peak or not, etc. These observations should be included in the “data, observations and results” section of your lab report.
· Use the linear diameter scale (lined up east-to-west except for Montes Apenninus) to measure the size of the object. Record this size to the nearest 1/10 tic.
· Repeat these observations and measurements for the objects on the list.
· TURN OFF THE LED ON THE ASTROMETRIC EYEPIECE and return your equipment.
Questions
1. Make a table of your measurements (of the tic marks) for each object. In the table also include the longitude of the object, its angular size, and its size in km.