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Ask the Astronomer
posted: 06:10 pm ET
21 August 2000
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Ask the Astronomer | Welcome! Ask the AstronomerForum is hosted by Jeff Kanipe. |
Welcome to our newforum, where you get answers to all your amateur astronomy questions. Howcan you get your question answered in this forum? Sendhim e-mail.
 YourQuestion: I know that the sun is about 150 million kilometersfrom the Earth. On your program it also said that it was 1,014 AU! Whatdoes the "AU" stand for? Is it "Astronomical Unit," and how far his 1 AU?
-- Patrice | JeffKanipe's Answer: The sun has a mean distance of 150 millionkilometers (93 million miles) and that distance is what defines a singleastronomical unit, or AU. The distances of all the other objects in thesolar system, then, can be defined using this handy gauge. Mars, for example,is 1.5 AUs from the sun; Jupiter is 5.2; Mercury is less than 1 AU at 0.3.
As for the 1,014 AU, that amountsto 152,100,000,000 kilometers (94,302,000,000 miles). This is a realm farbeyond Pluto among the Kuiper belt of comets, a disk surrounding the solarsystem containing upwards of a billion comets. In other words, way outthere! Related to this: | YourQuestion: I was just wondering about the distance measurementAU. and how they compare to miles light-years etc.
-- Thanks, Dan | | JeffKanipe's Answer: A single light-year's distance(about 6 trillion miles or 9.7 trillion kilometers) should be equivalentto 64,516 AU. |
| YourQuestion: Hi, where canI find Taurus in the Northern Hemisphere? Thanks! -- Mahendra | | JeffKanipe's Answer: Taurusthe Bull is a winter constellation, so at this time of year you need toarise during the wee hours to see it. The figure's distinctive V-shapedpattern of stars, with bright orange Aldebaran marking the Bull's eye,clears the eastern horizon around 1 a.m. Or, you could stumble out of bedjust before dawn, when the constellation is halfway up in the eastern sky.Note that the two bright "stars" just above the Bull's face are Jupiterand Saturn. |
| YourQuestion: "I would like to know how fast the Earth travels inspace? Also, how fast do falling stars fall?" -- Patrice Boucher | | JeffKanipe's Answer: In both cases, pretty darn fast, Patrice! Earth has a mean orbital velocity of66,600 miles (107,180 kilometers) per hour. So in a single day, Earth movesfurther along in its orbit by about 1.6 million miles (2.57 million kilometers).Owing to Kepler's laws of planetary motion, the Earth picks up speed alittle in the Northern Hemisphere's winter, when it is closest to the sun,and slows below the mean velocity when it is furthest from the sun in thesummer.Meteors aren't sluggards either. Theslow ones enter Earth's atmosphere at a velocity of about 7 miles per second(11 kilometers per second) while the fast ones blaze through at 45 milesper second (74 kilometers per second). The difference in velocity has todo with whether the Earth is catching up to the meteoroid when it entersthe upper atmosphere, in which case the speed is slow, or whether theyare on a direct collision course, in which case the speed is fast. |
| YourQuestion: Aloha, I live Upcountry on Maui, and being that weare 21 degrees north of the Equator I would assume I am looking at a slightlydifferent night sky than you are on the Mainland. Can you tell me whatto look for and where? Also, how far south of the Equator do you have tobe to see the Southern Cross? Thanks! Sincerely, Sarah Howard | | JeffKanipe's Answer: From latitude 20.40 degrees N., which is closeto the latitude of the Upcountry of Maui, approximately a 20-degree swathof southern sky that is not visible from latitudes 35 degrees N is plainlyvisible a bit more if you live above sea level. This region is extremelyrich in stars, star clusters, and nebulas. Dominating your southern view tonightis the Milky Way in the Scorpius/Sagittarius region. What makes this regionso unique is that it lies in the direction of the center of our galaxy.If you turn a pair of binoculars on this region you'll see a rich fieldof glittery stars and numerous open clusters, like M6 and M7 off the scorpion'stail. On the northwestern border of Sagittarius, are two bright nebulasknown as the Trifid and the Lagoon. The Trifid lies north of the Lagoon,and in binoculars looks a grayish patch of light. Closer inspection witha telescope reveals that three lanes of dark obscuring dust subdivide thisobject. The Lagoon is the larger and brighter of the two. A small telescopeboosts the contrast enough to show that it is bisected by a broad dustlane the "lagoon" from which the nebula gets its name. Below Scorpius you should be able tosee the southern constellations Ara the Altar, Norma the Square and, justbeginning to rise in the south-southeast, Pavo the Peacock. These figuresnever make it fully above the horizon at more northerly latitudes. As for the Southern Cross, or Crux,as it is more formally known, this popular constellation lies closer tothe south celestial pole, so it's not going to rise very high in the skyfrom latitude 21 degrees. On the equator itself, Crux's greatest altitudeis about 26 degrees. From Maui, the Southern Cross standshighest during mid May evenings around 9:30. Still, you will need to look8 or 9 degrees above your southern horizon. You can also see the SouthernCross rise in the morning sky around December 20 at 4:30 a.m.
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