by Kim Malville
What’s happening in April
Venus is the brilliant object in the western skies this month, remaining visible for slightly more than three hours after sunset. Jupiter, a little fainter than Venus, lies in Cancer, between the constellations of Gemini and Leo. It is due south at dusk and stops moving retrograde on April 18.
April 4: There will be a lunar eclipse in the morning just as the full moon is setting in the west and sun is rising. This eclipse occurs 2 weeks after the solar eclipse of March 20 (see below): full moons and new moons are two weeks apart. The partial eclipse starts at 4:15am MDT and the full eclipse starts at 5:54am MDT. It should be a fantastic sight watching the eclipsed moon setting in the west.
April 10-12: Venus will be close to the Pleiades in the western sky.
April 19: A very thin crescent moon low on the western sky soon after sunset: to its right will be faint Mars and Mercury. Above the moon is Venus. The moon continues to move during the next few days upward toward and past Venus and Aldebaran. Note the red color of Aldebaran contrasted to the silvery white Venus. Aldebaran will be twinkling while Venus will be steady.
April 22: Lyrid meteor shower (meteors radiate out of the constellation of Lyra); best viewed from 11pm to dawn.
April 27: Look to the moon and just above it find Regulus, the brightest star of Leo the Lion.
April 30: Just after sunset look to the north-west for the planet Mercury. This week and the next will be the best chance for viewing it in 2015.
The solar eclipse of March 20
A once-in-many-lifetimes solar eclipse occurred over Europe last month. Lots of people saw the sun partially eclipsed, but only those who braved the cold in Denmark’s Faroe or Norway’s Svalbard islands had a chance to see the full spectacle of a total eclipse. It was unusual in that it occurred at equinox. At the North Pole, the sun would be finally appearing on that day after six months of darkness. But, soon after it appeared it would disappear. That happens once every 400,000 or 500,000 years at the Pole. What a bummer for those polar bears! Some of the best viewing was near the town of Longyearbyen on Svalbard, where its normal population of 2000 was doubled for the eclipse, which lasted all of 147 seconds. The town was worried that there would not be enough food for the visitors and that there might be polar bear attacks.
So, you might ask, why go to all the effort of traveling to a remote location on the earth when satellites are capturing images of the solar corona from space? The answer is that only ground-based telescopes give pictures of the very fine and complex structures of the corona, such as in this photograph from the eclipse of 2008. We still do not understand why the corona is so hot, a whopping two million degrees compared to 6000 degrees on the sun’s visible surface. The streamers in the photograph are caused by wiggling magnetic fields, which somehow heat the gas. The corona has been an elusive puzzle for centuries and perhaps even millennia. The pearly glow which briefly appears around the sun when it is covered by the moon is an astronomical will-o’-the-whisp goal for astronomers like me, as we search for understanding of its mysteries, which we have yet been able to reach.
The oceans of Ganymede
Ganymede is the largest moon of Jupiter and also the largest moon in the solar system, larger even than the planet Mercury. It contains an iron core, a magnetic field, and flickering aurora in its atmosphere. Now it appears to have one of the largest oceans of the solar system, buried beneath a thick crust of rock and ice. Recent observations with the Hubble Space Telescope suggest a subsurface, salt-water, electrically conducting ocean, which contains more water than all the oceans of Earth. Here is another potent niche for life in the solar system. We already know about oceans underneath another Jovian moon, Europa, and a large lake beneath Enceladus, a moon of Saturn. The ocean of Ganymede appears to be 60 miles thick—10 times deeper than Earth’s oceans—and buried under some 95 miles of ice. What wondrous creatures may be swimming in the dark waters of each of these three moons?
Ground Hog Day in the sky
Bill Murray in the 1993 movie was trapped in reliving Ground Hog Day over and over again. Now astronomers are watching the multiple replays of a supernova that destroyed itself more than nine billion years ago. These replays are coming to us thanks to the curving of space, first suggested by Albert Einstein 100 years ago. Einstein would have been 137 years old on March 14, and these reappearances of the supernovae are fine birthday tributes to the man. Actually, these observations by the Hubble space telescope are wonderfully amazing confirmations of ideas about General Relativity, which include the bending of space due to gravity and the slowing down of time by gravity. This is all the more wonderful because there is no such thing as a “Force of Gravity.” Our experiences of gravity are entirely the result of curved space. No force is involved. In the case of this supernova its light rays have been bent around a massive galaxy, lying between it and us, so that multiple images of it keep arriving. There are four images of the exploding star, each the result of a different path for the rays of light. Einstein proposed that matter and energy warp the geometry of space the way a heavy body sags a mattress, producing the effect we call gravity. One consequence of this is that light rays are bent and follow curved paths around massive objects such as stars and galaxies, an effect first confirmed during a solar eclipse in 1919.
This supernova, named after the Norwegian astrophysicist, Refsdal, has probably not finished sending us messages. We expect another image to arrive sometime in the next ten years. But hang on, it gets even stranger. Because of the expansion of the universe, the star and its galaxy are receding from us so close to the speed of light that clocks on the planets of that galaxy (there must be a few clocks) are ticking more slowly than those on Earth. As a result, one month for the residents of that galaxy corresponds to nearly three months on Earth. The explosion of supernova Refsdal is appearing to us in slow motion. All I can say is wow! What other mysteries await us in the universe? Any questions? Try: email@example.com.