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
"Happy Pi Day!
THE DAILY ASTRONOMER
Monday, March 14, 2016
Hotter Sun and the Last Solar Eclipse
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ASTRONOMY CAFÉ: "The Distance to the Sun"
Monday, March 14, 2016 7:00 p.m.
at the Southworth Planetarium
How did a transit of Venus help astronomers determine Earth's distance from the Sun? Join us for an informal discussion about how we figured out that our parent star is about 93 million miles away.
Admission by donation
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"Radio Astronomy" Friday afternoons at 1:00 p.m. on WMPG 90.9 FM www.wmpg.org
This coming Friday, we open Pandora's Jar and answer as many astronomical questions as possible.
Do you have an astronomical question you'd like us to answer on the air?
Just send your question to [log in to unmask]
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Last week, we ignored Pandora's Jar altogether. This jar, we probably should mention. is the vessel containing the parchment paper slips on which we've written subscriber questions. We would that we could answer them all at once and not leave some to languish within the jar's hollows. However, time permits us only to visit it periodically.
Today, we've removed two more questions.
The Alpha Pandora question pertains to the Sun's increasing temperature.
The Beta Pandora question involves the future of solar eclipses.
"Is the Sun getting hotter? If so, why? Will Earth eventually become too hot for life?"
-Sean S, Biddeford
The Sun is becoming increasingly hotter or more luminous with time. However, the rate change is so slight we won't notice anything even over many millennia, let alone a human life time. Eventually, however, the Sun will become so luminous that it will render Earth inhospitable to life.
Let's work through some of the science:
The Sun produces energy through core thermonuclear fusion reactions which converts hydrogen into helium. These reactions generate copius energy that slowly migrates out toward the photosphere and then into space. Astronomers believe that the Sun formed approximately five billion years ago, at which time it initiated these reactions. They have continued ever since.
As the Sun ages, it slowly grows hotter due to the accumulation of residual energy emitted by these core reactions. Initially, the Sun was only about 70% as luminous as it is today. Consequently, the solar constant, the energy Earth receives from the Sun, would have been correspondingly lower. "The Faint Sun Paradox" was borne out of the realization that, though the Sun was cooler in its infancy, the early Earth still contained liquid water. * Throughout the subsequent billions of years, the Sun's luminosity increased gradually and will continue to increase in the future. Astronomers estimate that the Sun's luminosity will increase about 6% every billion years. This increase might seem slight, but it will render Earth inhospitable to life in about 1.1 bllion years. The planet will be too hot to support life. When stellar astronomers first understood the Sun's energy generation mechanism, they believed that Earth life would surive until the Sun expanded into the red giant stage. Today. they know that our time is much shorter, albeit still more than one billion years.
"I watched -on a live feed- some of the total solar eclipse that happened earlier this month. I wondered, if the Moon is moving away from us, will there come a time when there are no more total solar eclipses?"
-Maureen M, Portland
Total solar eclipses occur when the moon moves directly in front of the Sun. The moon can completely block the Sun because the angular dianeters of both are about one half a degree. Although the Sun's dianeter is about 400 times larger than the moon, the moon is about 400 times closer to us than the Sun. Consequently, they appear to be about the same size. However, as you mention, the moon is receding from Earth. The current rate is about 3.8 cm/year** Though its recession rate is slow, the moon's angular dianeter will decrease slowly so that eventually, the moon's angular diameter will be smaller than the Sun's. At this point, the moon will still be able to move in front of the Sun, but the resultant eclipses will be "annular," not total. During an annular eclipse, the moon only blocks a portion of the Sun. A "ring of fire" remains around the moon, hence the term "annular," meaning "ring-like."
We can and so have annular solar eclipses in our time. They occur when the moon is at or close to apogee, its most distant point, during an eclipse. Mathematical astronomers estimate that in about 600 million years, Earth will experience only annual solar eclipses.
The next total solar eclipse visible in the USA occurs on August 21, 2017!
While we know that total solar eclipses won't last forever, there will be millions more in future.
*Astronomers and geologists are still attempting to reconcile this paradox. Some suggest that Earth's atmosphere was much thicker in its youth and contained greater quantities of heat retentive carbon dioxide. Such a gaseous envelope would have been able to retain more heat onto the planet's surface, just as Venus' carbon-dioxide rich atmosphere does today. Also, the Moon was closer and therefore the resultant tidal heating was much greater, as the tidal forces induced by a nearby body are very distance sensitive. These and other factors could explain how a cooler Sun could have sustained a warmer Earth.
**One might wonder how scientists can know this value so precisely. They know how the moon's distance changes because of the Lunar Laser Ranging Experiment. Astronauts from Apollo missions 11, 14, and 15 installed retroreflectors on the lunar surface. Earth bound scientists focus laser beams toward these reflectors which they send them back to our planet. By timing the laser pulse's return, they can quite accurately know the moon's distance.