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.