THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N, 70.2667° W
Founded January 1970
"We'll leave the night on for you."
THE DAILY ASTRONOMER
Tuesday, April 19, 2016
Outrunning the Night and Other Ideas
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STAR DOME ASTRONOMY!
Our introductory astronomy class begins tonight.
Walk in registrations welcome.
Class begins at 7:00 p.m.
Doors open at 6:15 p.m,
Enrollment: $70.00 ($50.00 for members)
For more information, consult our web-site: http://usm.maine.edu/planet/star-dome-astronomy-level-i )
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This week, we are bound, gagged and determined to sort through the pile of parchments now packed in Pandora's Pouch. Well, it's actually a jar, but using that word would have messed up the alliteration. Each piece of parchment contains an astronomical question that someone sent to us. We store these questions to be answered at a later date. We generally devote one day a week to addressing these queries. However, during this rather busy week, we decided to answer questions each day this week, apart from Friday, which, of course, is quiz day.
Please continue sending questions!
For an archive of past questions and answers, consult our "Ask the Staff Astronomer" web-site: http://usm.maine.edu/planet/ask-staff-astronomer
"I remember reading a science fiction story about Moon colonists who were able to out run the night. The night-line moved across the lunar surface more slowly that they were running, so by running, they could remain in daylight. Is the science behind this idea right?" Martha P, Michigan.
Well, this is a very interesting idea: outrunning the night-line. The answer is a bit involved, so we will address this one in stages.
First, the lunar line of demarcation separating daylight and night is called the terminator. As the moon rotates on its axis, this line advances across its surface, just as as Earth's "terminator" constantly advances around the planet. Earth, however, possesses an atmosphere, which refracts the sunlight, causing the twilight phases which follow sunset and precede sunrise. The moon lacks an atmosphere, so its terminator is sharply defined.
Determining how fast this terminator moves requires some math, but we'll be gentle.
First, the moon's rotates once on its axis every 29.5 Earth days.
The moon's equatorial circumference (the distance around the moon's equator) is about 10920 km (6770 miles). Now, 29.5 days equals 708 hours. So, at the lunar equator, the terminator advances at 15.4 kilometers (9.5 miles) per hour.
Running that quickly requires some stamina. The world record for a marathon completion time is 2:02:57 by Dennis Kimett. (Surprise! He's from Kenya). To run a marathon that quickly, Dennis had to run an average speed of 12.9 miles per hour. So, runners who are as fast as Dennis Kimett and other fast marathon runners could outrun the lunar terminator on the equator.
Realize, that the 99.99999999999% of Earth's runners who are slower could outrun the terminator at higher latitudes. Points on the moon at increasing distance from the equator move at reduced speeds. For instance, at latitude 45 degrees N or S on the Moon, the terminator is moving at 6.7 miles per hour. At 66.5 degrees N or S, the terminator speed is about 3.8 miles per hour. A rapid walker could outpace the lunar terminator at its higher latitudes.*
"I am already hearing about the total solar eclipse that America will experience in August 2017. Will the solar eclipse be total in Maine?"
-No, unfortunately.
We will see the Sun partially eclipsed in August 2017. Here, we'll see about 65% of the Sun blocked by the moon during this eclipse. Mainers will next see a total solar eclipse in 2024.
"The Mercurian Transit module mentioned that Venus and Mercury would transit the Sun simultaneously sometime in the very distant future. I wondered, can a transit of Mercury and transit of Venus happen in the same year? Or, can a single year have two Mercurian transits or two Venusian transits?" -S.F., South Portland.
Yes, transit of Mercury and transits of Venus can happen in the same year, but such years are rare. The last year this happened was 1769. The next time we'll have two transits will be 2611. No single year can ever have two transit of Mercury or two transits of Venus.
*To calculate the terminator's speed, multiply the equatorial speed (9.5 mph) by the cosine of the latitude. To see an example of this calculation for Earth, consult the planetarium's "Math Zone" web-page: http://usm.maine.edu/planet/mz4-determining-speed-earths-rotation-given-latitude ''