Lunar Eclipse Observations. A lunar eclipse will be occurring on the night of February 20. That will be your one and only chance to do this exercise.

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1 Lunar Eclipse Observations A lunar eclipse will be occurring on the night of February 20. That will be your one and only chance to do this exercise. Materials: Paper, pencil, compass (or string, and pin or another pencil). Read through these materials ahead of time. You won t be able to read during the eclipse itself, and if you miss something, there s no second chance. You will observe the Moon while it is in eclipse, and as it enters and exits the eclipse. Make sure to have one observation during the total phase of the eclipse. You will draw the edge of Earth s shadow on the Moon, and use that to estimate the relative sizes of the Earth and Moon. You shall also estimate the time for the Moon to pass through Earth s shadow, and the darkness of the shadow. The Danjon scale described at the end of this write up is a standard way to estimate the color of Earth s shadow. The shadow is not totally dark, because some sunlight is bent by Earth s atmosphere so that it hits the Moon. Earth s atmosphere and pollution remove much of the light, mostly the blue light, leaving the Moon colored anywhere from rust to dark grey. Because the amount of light differs for each eclipse, observers (such as you) are asked to estimate the color. What to do: The upcoming eclipse will be total, that is, the Moon will be entirely covered by Earth s shadow. Look at the figure attached to the back of this writeup to see the important times. If you miss some of it, you cannot get full credit! Note that sunset is scheduled to occur at 5:52 pm on February 20, and remember that the Full Moon rises at sunset. Because light is bent by Earth s atmosphere, the Moon may be just visible above the eastern horizon as the partial phase of the eclipse begins. 1) During the initial partial phase of the eclipse (between 5:43 and 7:01 pm), sketch the Moon with Earth s shadow on it. Don t sketch it either very early or very late during the partial phase. It will be very hard to measure the curve of the shadow at those times. You ll do best when the shadow covers about half of the Moon. You can use one of the circles on the pages at the end of the exercise. Use the circles to represent the Moon, and indicate the arc of the shadow. Write the time that you made the measurement on each sketch, and indicate the direction of the horizon with an arrow. Do this once or twice. 2) Observe the Moon while it is in the total phase of the eclipse. Use the Danjon scale described below the to describe its appearance. Enter your data in the table below. You will not be able to see the edge of Earth s shadow. 3) Look at the time either when the Moon just completes entering the darkest part of Earth s shadow, or when it just begins to leave the darkest part of the shadow. You can

2 tell this when you either no longer see any of the Moon looking relatively white, or just start to see a white sliver on the Moon again. Record this time in the table below. 4) As the Moon leaves Earth s shadow, repeat the measurement that you made in part 1). Again, do it once or twice, for a total of three measurements. 5) Now, in the comfort of home, trace over the shadow that you drew with a compass. If you don t own a compass, use some string, a pencil, and a tack as shown Adjust both the length and location of the string (or width of the compass) until the pencil traces out the circular arc of Earth s shadow across the Moon. The length of the string, or the width of the compass, is then the radius of the shadow. Repeat this for all three of your sketches. Enter the times and radii in the table. Note that the shadow will have a much larger radius than the Moon, and so the compass or tack will be centered off of the sheet where you have made your sketch. 6) Average the three values of the shadow s radius, and enter the result in the table. Divide that number by the radius of the circle that represents the Moon. That ratio is close to the actual ratio of the radii of the Earth and Moon. Look up the actual values of the diameters of Earth and Moon from your textbook. How does the true ratio compare to your observed value? Give your result as a percentage error (note that in reality, Earth s shadow at the Moon s distance is actually about 25% smaller than the Earth, and so your results can t possibly agree exactly). 7) Now, use the time that you recorded to estimate your longitude. Totality begins at 3:01 am, and ends at 3:51 am, February 21, Greenwich Mean Time (GMT). Greenwich time is a standard reference for timekeeping. For historical reasons, standard time was originally kept at the port of Greenwich, England, and that time is used today when we need to refer to a standard time that can be referred to by astronomers who are in different time zones. Because we are in a different time zone than Greenwich, England, we see the eclipse happening at a different time. Assuming that your watch is reasonably accurate, you will notice that the times you have recorded don t exactly agree with those on the figure attached to the end of this exercise. That is because we are not exactly at the center of the Pacific Time Zone. To find your latitude, first determine the difference between your measured time for entering or exiting totality, with the corresponding time in GMT given above. Convert differences in minutes to fractions of an hour (for example, 15 minutes is 0.25 hours). A

3 difference of one hour corresponds to 15 degrees of longitude (west longitude for us). Now, look up our latitude (on-line is probably the easiest method). How far off are you? 7) On a separate sheet of paper, answer the following questions: 1) Why is the shadow of Earth a circle? 2) What do you think is the main cause of discrepancy between your value for the Earth/Moon radii ratio, and the accepted value? 3) What do you think is the main cause for your error in your latitude measurement? 8) Hand in your sketches, tables and answers to the above questions. Be sure to write your name on the paper!

4 Table 1: Data for total eclipse Time Shadow Brightness on Danjon Scale Your description of the shadow color/brightness Table 2: Data for partial phases Time Radius of Shadow (in centimeters) Average radius of Earth s shadow Ratio of Earth/Moon radii, based on your observations Ratio of Earth/Moon radii, from your text Percentage error in your value Your value actual value % error = x 100 actual value Table 3: Time and latitude Time for beginning/end of totality Difference between your time and GMT Your measured longitude Your actual longitude

5 Date: Time: Date: Time:

6 Date: Time:

7 The Danjon Scale of Lunar Eclipse Brightnesses (From Fred Espenak, writing for Sky & Telescope magazine) The Moon s appearance during a total lunar eclipse can vary enormously form one eclipse to the next. Obviously, the geometry of the Moon s path through the umbra plays an important role. Not as apparent is the effect that Earth s atmosphere has on eclipses. Although the physical mass of Earth blocks off all direct sunlight from the umbra, Earth s atmosphere refracts (bends) some of the Sun s rays into the shadow. Earth s atmosphere contains varying amounts of water (clouds, mist, precipitation) and solid particles (dust, organic debris, volcanic ash, smog). This material filters and attenuates the sunlight before it s refracted into the umbra of Earth s shadow. For instance, large or frequent volcanic eruptions that inject large amounts of ash into the atmosphere are often followed by dark, red eclipses for several years. Extensive cloud cover along Earth s limb also tends to darken the eclipse by blocking sunlight. The French astronomer, A. Danjon, proposed a useful five point scale for evaluating the visual appearance and brightness of the Moon during total lunar eclipses. The L values for various luminosities are defined as follows: L = 0: very dark eclipse Moon almost invisible, especially at mid-totality L = 1: dark eclipse, gray or brownish in color Details on the Moon distinguishable with difficulty L = 2: deep red or rust-colored eclipse Very dark central shadow, while outer edge of umbra is relatively bright L = 3: brick-red eclipse Umbral shadow usually has a bright or yellow rim L = 4: very bright copper-red or orange eclipse Umbral shadow has a bluish, very bright rim The assignment of an L value to an eclipse is best done with the unaided eye, binoculars, or small telescope near the time of mid-totality. It is also useful to examine the Moon s appearance just after the beginning and just before the end of totality. The Moon is then near the edge of the shadow, giving an opportunity to assign an L value to the outer umbra. In making any evaluations, you should note the instrument that you used, and the time that you made the observation. Also note any variations in color and brightness in different parts of the umbra, as well as the apparent sharpness of the shadow s edge. Pay attention to the visibility of lunar features within the umbra. Notes and sketches made during the eclipse are invaluable in recalling details, events, and impressions.

Reprinted From AAC Newsletter FirstLight (2008 Jan/Feb), Revised Feb. 18, 2008

February s Total Lunar Eclipse Howard L. Cohen Reprinted From AAC Newsletter FirstLight (2008 Jan/Feb), Revised Feb. 18, 2008 Unlike Florida s two prior total lunar eclipses, which could only be partially