Cumberland Times-News

Bob Doyle - Astronomy

May 18, 2013

Here are curious facts about Earth’s moon

Several weeks ago, we had our annual STEM Faire at Frostburg State University. STEM stands for Science Technology Engineering and Mathematics.

There were over 20 displays, most involving interaction between the visitors (mostly elementary students) and FSU students. I had a display on our Moon, featuring the latest moon globe, copies of an article I wrote on the moon as well as a moon quiz.

This column will feature an abridged version of my moon article and the moon quiz with an explanation for the answers. The abridged article will feature some clues that will aid you in doing the moon quiz.

After the sun, the moon is the brightest celestial object. The moon’s cycle of phases (growing to full, then shrinking to invisibility) takes 29.5 days.

Some calendars still use the moon’s phase cycle for their months. The problem with using the lunar cycle is that 12 lunar cycles are only 354 days long, 11 days short of our year length.

The Jewish calendar fixes this problem by adding an extra month, seven times over a 19-year period. So the dates of Jewish holy days vary from year to year.

People in coastal areas noticed how the ocean waters would rise and fall twice every 24 hours and 50 minutes. There are two ocean regions on the Earth where there a high tides (where the moon is highest and the region facing away from the moon).

The 50 minutes is due to the moon’s motion about the Earth, where the Earth has to spin an extra 50 minutes to bring the moon back into the same relative sky position.

The Earth is about 8,000 miles across with the moon being about a quarter as wide. If we represent the Earth by a standard globe (1 foot across), the moon would be a tennis ball 30 feet away.

The grey areas seen by eye are dark basaltic rock, similar to the material at the bottom of our oceans. The grey areas on the moon started with huge asteroid impacts that created large basins on the moon.

Cracks in the moon’s crust caused these basins to be filled with molten moon material. These dark areas form the ‘woman or man in the moon’ that some of us see when the moon is full. (May’s full moon is this Friday evening — Saturday morning, May 24-25.)

The Earth got clobbered too, but erosion and plate motion have obliterated nearly all of our terrestrial craters. There are traces of a large underwater crater at the entrance of Chesapeake Bay.

Not all full moons are the same, for the full moon is opposite the sun along the zodiac. In the late spring/early summer months, the sun has the highest sky path, shining about 14-15 hours.

The moon then has its lowest sky path (like the late fall/early winter sun), getting only 1/3 of the way up (from horizon to top of the sky) and being in view for 9 or 10 hours.

But when the sun has a low sky path in late fall, early winter the full moon then rides high across the sky, illuminating the cold nights.

Eclipses are like black outs, some occurring during the day and others at night. A solar eclipse occurs when the moon’s casts its narrow shadow on the Earth, turning day into night.

Total solar eclipses are rare. The next one in the U.S. will occur on Aug. 21, 2017 with the zone of totality (darkness) running from the Midwest, then into the southern states (such as Tennessee and South Carolina). In our Tri-State area, this will be a partial solar eclipse.

A lunar eclipse occurs when the full moon passes through the Earth’s shadow. It will then be visible from the entire night side of the Earth. There will be a late lunar eclipse on the night of April 14-15 in 2014, visible from our area.

 Below is the moon quiz.

1. If the Earth is about 8,000 miles across, how wide is our moon?  A) 2,000 B)3,000 C) 4,000 miles.

2. How far away is our moon from the Earth? A) 60,000 B) 120,000 C) 240,000 miles.

3. How often do the tides change (from high to low)? About every A) 4 B) 6 C) 12 hours.

4. At start of what season is full moon highest in the night sky? A) Autumn B) Winter C) Spring D) Summer

5. About when does the full moon set?  A) Midnight B) Sunrise  C) Noon

6. What causes an eclipse of the moon? A) Moon goes into Earth’s shadow B) Moon’s dark side faces us

7. What causes an eclipse of the sun? A) Moon’s shadow touches Earth B) Earth gets into sun’s shadow

8. At moonrise why does moon look bigger? A) It’s closer B) We compare moon to horizon objects

9. What is a “blue moon”? A) Our blue sky haze makes moon look blue  B) It second full moon in a month

10. If moon is full on your birthday, when will moon again be full on your birthday? In A) 7 yrs. B)1 9 yrs.

  The answers are: 1–A, 2-C, 3-B, 4-B, 5-B, 6-A, 7-A, 8-B, 9-B, 10-B

   Some explanation: 1 The moon’s diameter is actually 2160 miles. 2. The average distance of the moon is about 239,000 miles (center of moon to center of Earth). 3. The tides along the coast change every 6 hours and 12 minutes. 4. The highest full moon occurs about the start of winter. 5. The full moon being opposite the sun, sets as the sun rises. 6. An eclipse of the moon occurs as the moon goes into the Earth’s shadow. 7. A solar eclipse occurs when the moon’s shadow touches the Earth’s surface.

8. The moon seems to be larger when near the horizon. This is a psychological effect called ‘the moon illusion’.

9. A ‘blue moon’ now means a second full moon in a month; this only happens with a very early and a late full moon in a particular month. ‘Blue moons’ occur about every 3 years.

10. The Jews learned from the Greeks that 235 lunar cycles very nearly equals 19 years. So every 19 years, the times of the moon’s phases happen at very nearly the same dates. The Jewish Holyday dates will also repeat.   

SKY SIGHTS AHEAD: Besides the moon growing to full on the coming weekend, there will be a fine grouping of three planets very low in the western dusk. Find a place with a flat western horizon.

Starting as early as 9 p.m., the planets should start to be seen. First to appear is the brilliant planet Venus. Second to appear will be the bright planet Jupiter. Third and dimmest is the planet Mercury.

You are advised to use binoculars to be sure of seeing Mercury. You can start looking on any clear evening in the second half of May.

The planets will change their relative positions from night to night. May 24 will have Mercury and Venus closest. On May 28, Venus and Jupiter will be 1 degree apart with Venus on the right. On May 22 the moon appears close to the planet Saturn in the southeastern evening sky (look above and to the left of the moon for Saturn).

Bob Doyle invites any readers comments and questions. E-mail him at rdoyle@frostburg.edu . He is available as a speaker on his column topics.

 

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Bob Doyle - Astronomy
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