by Kim Malville
Spectacular total lunar eclipse September 27
This will be a big one! A lunar eclipse occurs when the moon passes into the shadow of the earth. This particular moon will be a super-moon, when the moon is very close to the earth, at perihelion in its orbit. In fact, it will be the largest and brightest full moon to be eclipsed in your life time. The next total lunar eclipse will be on January 31, 2018 and will not be as good as this one.
A lunar eclipse reveals the shadow of the earth as well as the relative sizes of the earth and moon. The sun is behind the viewer and is casting its shadow outward into space. The full moon is always opposite the sun in the sky, and as it moves across the sky it gives us an awesome view of the earth’s shadow imprinted upon it.
Sometime around 350 BCE Aristotle, believing that a lunar eclipse occurred because the moon had entered the earth’s shadow, concluded the earth could not be flat because of the curved shadow it cast on the moon. It could be a sphere or, if you are a member of the flat earth society, it could be a round disk. Then, nearly a hundred years later, Aristarchus of Samos noticed that shadow of the earth is less rounded than moon itself. He concluded that if the sun is very far away, the earth must be about 3 times larger than the moon. He was incredibly close. The actual value is 3.7. This was a major breakthrough in humankind’s understanding of our cosmos: big earth, small moon, distant and very big sun. That was not bad for 23 millennia ago. A few years later Eratosthenes, a younger contemporary of Aristarchus, measured the diameter of the earth by comparing the shadows of the sun in wells at Alexandria and Syene in Egypt. That was the end of the flat earth. It had to be a sphere.
Back to our eclipse. The total eclipse will last 1 hour and 12 minutes, beginning at 8:11 in the evening and ending at 9:23. The partial phase will start at 7:07 when the edge of the dark center of the earth’s shadow, the umbra, begins eating one lunar crater after another. It will take just over an hour for the moon to be fully consumed. The moon may become increasingly foreboding, in its blood red color. The shadow contains the light that has skimmed the earth’s atmosphere and has been bent inward. It is the light of all the sunset and sunrises of the earth. Pretty wonderful, when you think about it. An astronaut standing on the moon would see the earth surrounded by an orange glow, which would be strong enough to color the surrounding surface of the moon.
Autumn starts September 23
The sun crosses the celestial equator on September 23. Equinox occurs on 2:21 am Mountain Daylight Time.
Seven hours after its July 14 closest approach to Pluto, the New Horizons spacecraft looked back and captured this spectacular image of the dwarf planet’s remarkable atmosphere, backlit by the sun. The image reveals layers of haze as high as 80 miles above its surface, several times higher than predicted. This is much different from a solar eclipse viewed on the earth. Our moon is dry rock and has no atmosphere, because of the way it was formed. The moon was produced by a fiery collision between the earth and a wandering planetoid the size of Mars. The heat of that collision evaporated volatile compounds such as water or methane from its surface. Pluto, on the other hand, was formed in the outer solar system, and contains the icy compounds that existed in the early solar system, far from the heat of the sun.
“My jaw was on the ground when I saw this first image of an alien atmosphere in the Kuiper Belt,” said New Horizons Principal Investigator Alan Stern of our Southwest Research Institute in Boulder. “It reminds us that exploration brings us more than just incredible discoveries–it brings incredible beauty.”
The haze in Pluto’s atmosphere forms when ultraviolet sunlight breaks apart methane gas, which evaporates from its surface due to heating from the sun. The breakdown of methane produces complex hydrocarbon gases, such as ethylene and acetylene. As these gaseous hydrocarbons fall to the lower, colder parts of the atmosphere, they condense into ice particles, forming the haze.
Rosetta’s Comet is heating up
Last month the European Space Agency released images of comet 67P taken by Rosetta from about 100 miles away, showing jets of dust and gas streaming from the nucleus. Although beautiful and dramatic these plumes of dust and gas make it hard to navigate the spacecraft. The space craft stabilizes itself and navigates using images of stars. The small telescopes that lock on to stars are confused by bright particles ejected from the comet.
The comet reached perihelion last month when it was 186 million km from the Sun, placing it between the orbits of Earth and Mars. Perihelion is an important event in the life of a comet, when it is subjected to the greatest amount of solar radiation, and it releases the largest amount of gas and dust. These cometary jets produce the magnificent glowing tails of comets and also drop dust particles along its orbit. When the earth passes through certain of these dust stream, we get meteor showers such as that of the Pleiades, which we experienced last month. This particular perihelion is especially exciting because this will be the first time a spacecraft has followed a comet to this state of maximum activity. It has always been a puzzle how and where the eruptions of gas and dust occur on the comet’s surface. The surface is coated with a dark tar-like substance, and these jets appear where there are gaps in the coating.
As you may remember the nucleus of the comet looks like a huge rubber ducky, with a mass of some 10 billion tons. The mission is planned to continue until September 2016. The managers of the spacecraft hope that during this time they might even be able to land it on the comet’s surface, which would be another space spectacular. Usually, the jets are faint compared to the nucleus, but this one in the photograph is brighter than the nucleus. This jet appeared suddenly and was gone after 18 minutes. Wow! Never before have we gotten such an intimate view of such an alien body. And then, to touch its surface and get really intimate will be the crowning climax.