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
Without change, there would be no vending machines."207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Founded January 1970
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On the next episode of: "WMPG: RADIO ASTRONOMY"
"Re-Opening Pandora's Jar!"
Friday, June 17, 2016 at 1:00 p.m.
WMPG 90.9 FM
We answer questions posted by planetarium patrons, DA subscribers, and "WMPG: Radio Astronomy" listeners.
For more information about the show (and to listen to archived broadcasts):
http://usm.maine.edu/planet/radio-astronomy-radio-program
http://usm.maine.edu/planet/radio-astronomy-radio-program
During last week's episode, Southworth Planetarium director Dr. Jerry LaSala discussed the Antikythera Mechanism:
http://www.wmpg.org/archive-player/?show_key=fri1300&archive_key=0
http://www.wmpg.org/archive-player/?show_key=fri1300&archive_key=0
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Earlier this week, Pandora's Jar certainly lived up to its reputation, for an answer pertaining to bullets elicited a barrage of responses! Ironically, the climate change response that we posted with some trepidation evoked nothing but silence. (Not necessarily a bad thing!) Thus do we illustrate our inability to predict what sort of response any article will provoke. We've been dipping into the jar quite frequently so as to now let it just overflow all over our pristine carpet.
Today, we answer two solar system related questions that subscribers sent us after having read previous DA articles.
"You said the Sun doesn't generally cross the meridian at noon time. It either does just before or after noon. The Equation is time tells us the correction factor. But, why doesn't the Sun cross the meridian at noon every day?" -Richard C, Portland
Mean noon rarely corresponds to true noon because, for one thing, Earth moves faster around the Sun in the winter than the summer because the planet is closer to the Sun in winter. (We remember that the closer a planet is to the Sun, the greater its orbital velocity.) However, Earth's rotation remains more or less constant throughout a given year. If Earth's orbital velocity were also constant, which would only happen if the planet's orbit were perfectly circular, the planet would progress along its orbit at a constant rate. Now, Earth advances faster in winter than summer.
A much more important factor is Earth's obliquity, or tilt. Earth is currently tilted relative to the vertical by 23.4 degrees. Therefore, as it revolves around the Sun, its polar alignment with the Sun constantly changes. Consequently, the Sun's apparent annual path through the sky, called the "ecliptic," is inclined relative to the celestial equator, the projection of Earth's equator onto the sky. If Earth weren't tilted, the Sun would appear along the celestial equator all year. Were it to do so, the Sun would progress in equal increments along its path and, if Earth's orbital speed were also constant, the Sun's diurnal motion would be more even and its passage along the meridian, (called 'upper culmination') would occur closer to noon and more frequently at noon, precisely.
This graphic shows the ecliptic inclined relative to the celestial equator. Because the Sun's path is so oriented, its daily progression relative to the equator will not be constant. At times, it will move more than one degree a day. At other times, it will be less than one degree a day. For this reason, the Sun will sometimes will cross the meridian before noon and at other times will cross the meridian after noon.
"Venus 'vanished' from our sky in April and won't return until late next month. Why does Venus go away from such a long time, but the planets like Jupiter and Saturn are only gone for about a month a year?" Courtney P, Bridgton
The only time we don't see "naked eye"planets is when they are too close to the Sun. When Jupiter or Saturn move "behind" the Sun, we cannot see them. However, these planets are so far from the Sun that they are orbiting very slowly. So, for instance, when Saturn moves too close to the Sun, Earth will quickly move into a position where Saturn becomes visible again. Think of this: You are walking around a telephone pole and much farther away another person is also describing orbits around the same pole. He is moving slowly, but you're walking quickly. At some point, the pole will appear to block the more distant person, but because you're orbiting quickly, he will soon be visible again. Now, assume that another person s closer to the pole than you are and moving slightly more quickly. When she is blocked by the pole from your view, she will not come back into sights as quickly because she will be moving rapidly and keeping herself out of your sight initially. Eventually, she will move back into your view more because of her motion than yours.
Similarly, Venus is moving along the far side of the Sun while Earth is also moving in the same direction more slowly. By the end of July, Venus will eventually re appear in the evening sky after a few months' absence. It is all part of the planetary dynamics that makes life so interesting.