Lunar Observations

Summary: Perhaps the most popular object in the sky, and one which provides a truly magnificent view through the telescope, is the moon. Its surface is covered with interesting features that reveal much about its history, and indeed about the history of the Earth-Moon system. To the naked eye, some of the more pronounced features visible as light and dark areas have been likened to the face of the "Man in the Moon." Binoculars and telescopes reveal countless craters on the surface of the Moon. These craters are the scars left by of ancient impacts by large meteorites, most of which occurred 3-4 billion years ago.

In the Moon lab, you will make observations of the moon, sketch features that you see, draw deductions about the moon based on your observations, and determine the dimension of the moon and of major features. This is a fairly long lab and may take more than one lab period to complete. The Moon is by far the celestial object with the richest details when viewed through a telescope. You will become familiar with the surface of the Moon by completing this lab. Take time to observe the countless details that it offers. The Moon may be a dead world, but it is a complex world nevertheless.

Note: You should make observations and sketches and to discuss what you are doing with your lab partner and classmates. The calculations can be done outside of lab time, based on the data you collect during lab. Similarly, you should discuss during lab time how to answer questions posed in this lab, but you should write down your answers outside of lab time. Everything goes into your observing log.

After you have completed the observations for this lab, answer the questions, based on your observations and discussion with your lab partner and classmates. Consult your your textbook for help and explanations if it includes a section on the Moon.

Near and Far Sides of the Moon: Refer to the map of the moon provided by your TA. Note that Earth-based photographs always show the same side of the moon. Because the moon's rotational period is the same as its period of revolution with respect to the stars (the sidereal period), only one side of the moon ever shows its face to Earth. This side is known as the near side. The other side, known as the far side, has only been seen directly by a few astronauts during the Apollo missions.

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Procedures

1. Date and Phase: Record the phase of the Moon as seen with the naked eye using the templates provided. Also draw the features that you see with your naked eye. Note the date, time and sky conditions.

2. With the telescope: Point the telescope at the moon and, with the 25 mm eyepiece, locate some of the features listed on the maps. The current phase may limit the areas that can be seen. Observe the smooth dark areas on the moon. Because the appearance of these regions reminded early astronomers of the smooth oceans, they are known as maria (the Latin term for "seas"). The rougher and brighter areas are known as highlands.

3. Maria and highlands: List all the maria that are visible. Contrast the smooth, dark maria with the light, rough highlands. The surface of the moon is composed mostly of basaltic rock, a typical rock formed from the cooling of molten lava. However, the basaltic rock of the maria contains a lot of iron, giving rise to the dark color. On the other hand, the highland basalts contain much aluminum. Because of this difference in composition, the maria have a greater density than the highlands. You don't have to sketch at this point but take good notes!

4. Terminator: Next, observe the terminator, the "line" separating sunlight from darkness on the moon. Switching to the 10mm eyepiece, draw a sketch of a small region of the terminator that you find interesting. Take the time to make a careful sketch that actually looks like what you see. Label the main features on your sketch.

   Question: Comment on how some aspects of what you see are different along the terminator than elsewhere on the moon. Describe and explain the differences.

5. Craters peaks: Identify one of the following craters: Agrippa, Delambre, Eratosthenes, Langrenus, Theophilus, or Tycho. In order to magnify the crater, replace the 25 mm eyepiece with the 10 mm eyepiece after centering. Draw a sketch of the crater and the mountain in the center of the crater, and any other features that may be physically associated with the crater. Take your time and do a good job!

   Question: Looking at the surface of the Moon in general, locate several craters with central peaks and several without. Is there a pattern between these two types of craters?

6. 6. Crater rays: Find one of the following craters: Aristillus, Copernicus, Langrenus, Kepler, Pickering or Tycho. Do not use the same crater you used in #5. Notice the bright rays radiating from the crater. These rays are much brighter a times near the full Moon.

   Question: What do you think may have caused these rays?

7. More Craters: Next locate either Plato, Facastorius, Ptolemy or Letronne. Again, magnify the image by viewing with the 10 mm eyepiece. and sketch the crater. You may see tiny craters peppering the floor of these large, flat craters.

   Question: In what way(s) is this crater different from the others you have been looking at. Notice especially the color on the inside of the crater walls. What other features on the moon share this same color?

8. Crater density and the relative ages of highlands and maria: One of the most important ways astronomers have learned about the moon, the history of the earth, and the history of the entire solar system is by ``crater counting,'' i.e. by comparing the numbers of small and large craters on different regions of the Moon and other bodies in the solar system. This can be done by counting the number of craters of various sizes on a given part of the Moon's surface. If youassume that the rate of crater-forming impacts on the Moon has remained constant since its formation, variations in crater densities imply variations in the ages of formation of the surface. Compare the crater densities of the highlands and maria and deduce which kind of terrain is youngest.

   Question: If you now consider that the rate of crater-forming impacts has decreased over time (is we know is the case), would the age of the highland and mare surfaces be closer to each other or even more different than under the assumption of constant crater formation?

9. How big is the moon? Measure the diameter of the moon using one of the two following methods:

   1. The method of transit times. This method works only if you can time a transit of the full diameter of the Moon, something that can be done only near the Full Moon or when the earthshine is fairly strong. Switch the telescope drive off and measure the time T (in minutes) it takes for the entire diameter of the moon to drift across one edge of the field. Also note the current declination of the Moon, d, based on its position in the sky and the SC001 star chart. Using the expression used in the field of view lab, obtain the apparent diameter the Moon, a (in minutes of arc):

      a=T cos(d) x 15'

   2. At other time the method of transit times would give only the thickness of the phase of the Moon, which is not particularly interesting in itself. If you can't use the method of transit times for a full lunar diameter, then you can use a somewhat less precise method based on the field of view of the telescope. Using the 25mm eyepiece and knowing its field of view in minutes of arc (see your results from the field of view lab), estimate the diameter of the Moon to the nearest minute of arc by simply comparing their relative sizes visually.

      The small triangle formula enables us to determine the diameter of the moon (d_Moon) if we know the angular size a (which we just measured) in minutes of arc and the distance to the moon (D_Moon), according to the formula:

      d_Moon = D_Moon x (a / 3438')

      Using radio signals, we know that the average distance between the center of the earth and the center of the Moon is D_Moon = 384,000 km. Provided that the diameter of the Moon is much smaller than 384,000 km, this formula will be a valid method for determining d_Moon. Note that the number 3438' corresponds to the number of minutes of arc in an angle of 1 radian and that 360^o=2*pi radian.

   Question: What is the diameter of the Moon, d_Moon in km?

   Question: Is the small triangle approximation we just used reasonable?

   Question: How does the diameter of the Moon compare with the diameter of the Earth and to the East-West extent of the contiguous United States? To do this, compute the ratio d_Moon / d_Earthand d_Moon/L_USA and comment.

   You can do the final calculation of d_Moon outside of class.

10. What shapes are craters? Select a few craters that are similar in angular size, selecting some near the middle of the moon, some part way to the lunar limb (the curved edge of the moon), some as close to the limb as you can find. Identify the craters and sketch their outlines only (i.e. general shape) and estimate how far away each crater is from the center of the disk of the Moon, using the apparent radius of the Moon as you unit of measure. For example, a crater at the center would be at a distance of 0, a crater halfway between the center and the limb would be at 0.5 and one right on the edge would be at 1.0. Put this number next to your sketch, along with the usual information (date, time, scale, orientation, sky conditions, etc).

    Question: Do the craters appear to change shape, moving from the lunar center outwards toward the limbs? If so how does the shape change?

    Question: Why should or shouldn't the apparent shapes of craters change or be the same as ne moves from the center toward the limb of the Moon?

    Question: What can you deduce about the shape of the moon based on these observations of apparent shapes of lunar craters?

11. More Observing. The table following the procedures lists some of the most interesting features visible on the surface of the Moon, including mountains, craters, rays, valleys and canyons. Become familiar with them by identifying and observing at least 8 features of various types. Takes notes on each of them (i.e. describe their appearance).

    Sketchin reasonable detail examples of three different types of features. Add to your sketch the dimension of each feature in minutes of arc and in km using the method of transit time to measure its apparent East-West size. Simply time how long the feature takes to cross the edge of the field of view (with the telescope drive off) and use the above formulas. Comment on the size (in km) of the features you measure.

12. The Lunar Limb. Now focus your attention to the lunar limb, the round edge of the Moon. Notice how the edge is not perfectly smooth. Make a sketch of a small portion of the limb that shows some jaggedness. Take notes. What are you really seeing?

13. Men on the Moon. Use the second table and the lunar map to identify at least two of the Apollo lunar landing sites. While no trace of the landings are visible through any Earth-based telescope, take a moment to look at the spot and reflect on these historical events that took place three decades ago. You can add personal thoughts and comments to your logbook.

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Craters, Rays & Mountains

Notes: Selenographic (i.e. lunar) longitude and latitude are defined in fashion very similar to their geographical equivalents. The meridian zero (longitude=0^o) is defined as the meridian that crosses (N-S) the middle of the visible hemisphere of the Moon. Selenographic latitude is measured from the lunar equator, which pretty much runs across (E-W) the middle of the disk. Thus, the central point on the lunar disk is at position (0E, 0N). East and West are defined for an observer standing on the Moon (cartographic convention, just like on Earth) and are reversed from the astronomical directions in the sky.

Name	Sel. Long.	Sel. Lat.	Comments
Langrenus	62E	-9	Large crater near the lunar limb
Petavius	60E	-26	Rima Petavius, running from the central peak to the rim is an unusual feature. Best seen when the Moon is young.
Messier & Messier A	47E	-2	This pair of craters probably formed simultaneously by a broken/binary impactor. Note the comet-like double ray extending from Messier A. Were these two craters caused by a grazing impact?
Proclus	47E	+16	Small crater with a very bright ray system on the edge of Mare Crisium.
Fracastorius	33E	-23	Flooded crater with collapsed N wall
Theophilus	26E	-11	Relatively young crater overlapping Cyrillus
Altai Mtns	25E	-25	Battered mountain range with sinuous scarp
Ariadaeus Rille	13E	+6	Long, straight ``canyon''
Cassini	5E	+40	Two smaller craters inside give it an unusual appearance.
Hyginus Rille	6E	+8	Long, narrow rille (``canyon'') with a bend and a superimposed crater (Hyginus). Terrain to the North has many smaller grooves.
Alpine Valley	2E	+49	Long, flat valley in the lunar Alps Mountains
Apennines Mtns	0	+18	Beautiful and complex chain of mountains
Mt. Piton	1W	+41	Isolated mountain peak in Mare Imbrium
Ptolemaeus	2W	-9	Large, round crater with dark floor. How many small craters can you see on its floor? Ptolemaeus is so wide (90km) that if you were standing at the center, you wouldn't see the crater rim as it would be below your horizon!
Archimedes	4W	+30	Large crater with flat floor
Rupes Recta	8W	-22	The "Straight wall" is a cliff that is 80 km long and 300 m high.
Mt. Pico	9W	+46	Isolated mountain peak in Mare Imbrium
Plato	10W	+52	Flat lava-flooded crater with dark floor. Can you see very small craters on its floor?
Tycho	11W	-43	Young crater with the most extensive ray system on the Moon.
Eratosthenes	12W	+14	Young crater at the S end of the Apennines Mtns. Compare to the larger Copernicus.
Stadius	14W	+11	"Ghost of a crater", flooded by lava during the formation of Sinus Aestuum. Only the upper part of the rim is visible.
Clavius	14W	-58	Very large, very old crater with many smaller craters superimposed (Seething Bay).
Montes Recti	20W	+48	Small chain of mountains at the edge of Mare Imbrium
Copernicus	20W	+10	Young crater. Note: central peaks, terraces inside wall, structure of outer slopes, chain of craterlets to the NE. About 60 km across, roughly the size of Davidson county.
Kepler	38W	+8	Very bright ray system, visible to the naked eye!
Gassendi	40W	-18	Floor criss-crossed by cracks
Schroter's Valley	50W	+55	Deep "canyon" near crater Aristarchus
Grimaldi	68W	-6	Large crater with flat, dark floor. Best seen near full Moon
High mountains	--	near South pole	This region shows the largest vertical relief on the Moon. These mountains and crater rims are seen sideways and reveal their true elevation. Compare with your perception of relief along the terminator.

Apollo Landing Sites

Mission/Date	Sel. Long.	Sel. Lat.	Comments
Apollo 11 (7/20/69)	23.5E	+0.7	Between craters Sabine and Moltke
Apollo 12 (11/19/69)	23.5W	-3.0	Between craters Fra Mauro and Lansberg
Apollo 14 (2/5/71)	17.5W	-3.7	Just North of Fra Mauro
Apollo 15 (7/30/71)	3.6E	+26.1	At the North end of the Apennines Mtns
Apollo 16 (4/21/72)	16.0E	-9.0	In the lunar highlands, W of crater Theophilus, just north of crater Descartes
Apollo 17 (12/11/72)	30.8E	+20.2	In the Taurus Mtns, near crater Littrow