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
Julian date: 2457681.16
"Nestled in night's smallest hollows."
THE DAILY ASTRONOMER
Tuesday, October 18, 2016
Double Pandora
It's like being a young, immortal fool and your mother's commanding you to clean your room sometime before the Sun goes supernova, for Heaven's sake! And, by the time you start explaining that the Sun isn't nearly massive enough to go......the door slams and you're left in isolation. As you look ruefully toward the door and miserably around at the contained catastrophe that only a workaholic microbiologist would like, you think that you have plenty of time to comply with the "request." Then, almost the next day, the Sun expands to a red giant and as the layers approach the incinerating Earth,you hear, "So, how's that room?!" And, before you can explain that what the Sun is experiencing isn't a supernova so you still have time, a stern woman is surveying your room with disgust and then surveying you with reproach....
Hence, our hundred-question Pandora pledge. We're only on parchment number 4 and it is already mid October. We have to have the hundredth parchment pressed and addressed by early August, which seems like it is a century and a half in the future, but will likely smack us broadsides as we walk through our gallery next week. (When one works in a dark planetarium, one experiences these temporal assaults from the past and future so often they become all too commonplace.) We have to curb this tendency to procrastinate (not that we're doing that now) and get on with it.
So, we're getting on with it.
Pandora Parchment # 4: The moon is tidally locked with Earth so that it keeps the same face toward the planet. I heard that eventually there will be a mutual tidal lock so that the Earth's rotation and the the moon revolution will be the same. When will this happen? -N.B, Freeport
You are correct that the moon and Earth will eventually achieve a mutual tidal lock, just as Pluto and Charon have done. However, this is not due to happen for about 50 billion years! The Sun will have ended its life cycle well before the moon and Earth reach this stage. To provide some background: The moon's rotation period equals its revolutionary period so that the moon directs its same side toward Earth. Meanwhile, as the moon revolves around Earth, it induces a "bulge" that is not exactly aligned with the Earth moon line due to Earth's rotation. The moon pulls on this bulge and the bulge pushes on the moon. As a consequence of the former, Earth's rotation slows down slightly: by less than a millionth of a second each year. (Those harassed workers who wish there were 25 hours in a day need only wait 140 million years for this to be true.) As a consequence of the latter, the moon gains a bit of energy, its orbit widens and it slowly recedes from us by a few centimeters per year.
However, the rate is so slow that the moon and Earth will not even come close to achieving complete tidal lock before the Sun's life cycle ends
Pandora Parchment # 5: How do we know the Sun rotates and is it true that the pole rotates at a different speed than the equator? -P. Spencer, Bath
The famous Italian astronomer Galileo Galilei (1564-1642) was the first person to have observed and recorded spots on the Sun, Curiously, we refer to these splotches now as "sunspots." Galileo observed that these spots migrated across the Sun, which was indicative of rotational motion. Moreover, this rotation time varies because the Sun is not a solid body, but is instead a fluid, or, more correctly a "plasma."
If the Sun didn't have spots, we would have been hard pressed to have known about its rotation. Galileo suffered the consequences of his observations, however, as he observed the Sun through his small telescope without any protection. During the last years of his life, Galileo was stone blind.
Rotating Sun. The Sun rotates on its axis. However, because it is a "fluid," it experiences differential rotation. The Sun's equatorial rotation rate is once every 27 Earth days, but its polar rotation is 35 Earth days. The rotation rate increases with ascending solar latitude. Image: Weather.gov
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FROM THE CATACOMBS OF INFINITE KNOWLEDGE
How fast are you moving, Earthling?
Unlike the Sun, Earth rotates as a solid body. However, the speed of rotation still varies with latitude because the circumference of any latitude circle, itself, is variable. It is maximum on the Equator and minimum at the poles. The Earth is nearly -though not exactly- spherical. Consequently, the distance around Earth decreases as one moves away from the Equator. Since every point on Earth completes a rotation at the same time as any other point, the rotation rate is determined by the size of the circle at a given latitude. A person standing on the equator travels more during one rotation than a person at a higher latitude, so the equatorial inhabitant has to travel faster.
It is the exact same notion as a spinning record! (Refer to the comic strip below the map.)
Earth's rotation speed. How fast are you moving on this rotating Earth? The answer depends on your location. People on the equator are moving at 1040 miles per hour. However, someone standing on the Arctic Circle (66.5 degrees N) is moving at only 414 miles per hour. The chart above shows the varying rotational speeds at different latitudes. Image: Seth Kadish
NASA launches rockets from the southern part of the United States because Earth is rotating faster there than at any other part of the country. NASA engineers, typically clever folks, have figured out how to impart a greater boost on these rockets due to the higher rotation speed.
Calvin and Hobbes by Bill Watterson