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
"Somewhere north of the undiscovered country."
*THE DAILY ASTRONOMER*
*Monday, July 11, 2016*
*Waiting for Juno and Full Moon Heat*
Now that we're within a few weeks of ending another Daily Astronomer school
year, we're already planning for the next one, which we intend to be the
most ambitious ever.* However, we first have to complete this year and,
in so doing, should clean out Pandora's Jar entirely. We should, but
likely won't as the queries continue, Mr. Lin, to come in at a fast and
furious rate. Though we're delighted by this deluge, we know that some
questions will sit in the jar throughout our summer holiday (Aug 3 - 21,
inclusive). We'll attempt to deplete Pandora as much as possible.
Please do not infer from this statement that we want this inflow to
cease. Quite the contrary. Don't hesitate to send your question or
questions!
Today's queries pertain to the spacecraft that just reached Jupiter and the
affect the full moon can have on our weather.
*"Why did it take Juno five years to reach Jupiter? Wasn't it moving
extremely fast?"*
-Kyle N, Brunswick
The answer is simply because Jupiter is quite far away. When the Juno
spacecraft established an orbit around Jupiter on July 4, 2016, this planet
was about 538 million miles from Earth. The Juno spacecraft launched in
2011 and in 2013, it accelerated around Earth using the gravitational
slingshot effect. This 'gravity assist' increased Juno's velocity
relative to the Sun from 78,000 miles per hour to 93,000 mph. By
terrestrial standards, this speed is impressive. However, by solar
system measure, the pace was sluggish. Remember that millions of miles
separate the planets even when they are at their closest.**
Recall the New Horizons spacecraft which reached Pluto last year after a
journey lasting nearly a decade! During most of its trip, the New
Horizons vessel was moving more than a million miles a day. The solar
system is mostly empty space with a smattering of planets and other bodies
widely dispersed within it.
*"The full moon shines by reflected sunlight and nights with the full moon
are very bright. Doesn't the moon reflect the Sun's heat, too? So,
would a full moon make a night warmer than it would otherwise be*?"
-C. McLaughlin, South Portland
Excellent question!
The answer is, alas, not as straightforward as I want it to be.
First, we know that the Sun heats the ground which then re-radiates the
heat to its surroundings, including the air. We also know that the Sun's
altitude determines how effectively it heats the ground. In the summer,
for instance, the Sun attains a high angle during the day. The solar
radiation passes through less atmosphere when it is high in the sky than it
does when the Sun is low. (There is 40 times as much atmosphere between you
and the horizon as exists between you and the zenith, the point directly
overhead.) In the winter, the Sun's altitude remains low even around noon,
and therefore the solar radiation passes through a thicker part of the
atmosphere, which absorbs a great deal of it.
The full moon's altitude also depends on the season, but in an inverse
manner. For instance, the full moon closest to the summer solstice will be
quite low, while the full moon around the winter solstice will be
high.*** The full moon's radiant heat will be greater in the winter than
the summer because of this angle difference.
Now, we need to determine how much heat the moon reflects back to us.
This issue pertains to the "albedo," the ratio of radiation reflected to
radiation received. For instance, the moon's visible albedo is 0.06,
meaning that it reflects about six percent of all incidental visible light:
a surprisingly small amount. However, albedo is "wavelength dependent," so
that the amount of reflected radiation depends on the wavelength of that
radiation: infrared, the 'heat' radiation and all the others. It turns
out that the moon's infrared output is 1/100,000 as intense as the solar
infrared radiation when the moon is full.
Taking all this into account, the full moon does raise the temperature of
the lower troposphere, where we're living, by about 0.03 degree F: a
minuscule, but still measurable amount. If you're the type of person
who loves to wander outside on a brisk winter night to observe the
scintillations of moon light above the crystalline snow fields -while us
sane humans are cramming our heads into heat ducts- realize that Diana's
orb is imparting heat onto you as well as light, although the former is far
more meager than the latter.
The full moon does affect the Earth's surface temperature, but only
slightly.
*A tour of all the Messier objects, more in depth sky calendars, a
plethora of pandoras, the return of long absent characters, the
introduction of new ones, explorations of exo planets, excursions into
distant galaxies, more mythology, more astronomy and, unlike this year,
not a single repeat….at least not until September 2.
**Earth and Venus can come closer to each other than any other two planets
in the solar system. Even the minimum separation distance between Earth
and Venus , which occurs around the time of inferior conjunction, is about
23.5 million miles!
***The full moon is, by definition, 180 degrees away from the Sun. The
Sun's annual path through the sky, called the ecliptic, is an undulating
curve with a high point marked by the summer solstice position and a low
point by the winter solstice position. The winter and summer solstice
points are separated by 180 degrees. The moon travels along a band
centered on the ecliptic and so, when the moon is full, it will be on or
close to the ecliptic position 180 degrees from the Sun. So, the full
moon on or close to the winter solstice will be located around the high
summer solstice point. The full moon on or close to the summer solstice
will conversely be around the low winter solstice point.