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."
*THE DAILY ASTRONOMER*
*Thursday, June 16, 2016*
*Explaining the Equation and Not Seeing Venus*
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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
www.wmpg.org
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
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
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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.