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Night Sky

March 2011 Night Sky

—Charlie Christmann

Astronomical Dictionary

It seems I have been remiss in defining many of the terms used to define astrological events. I would like to apologize to the students who e-mailed me to ask if I could define some words for them. So, here goes:

  • Conjunction: As seen from Earth, it is an event when two objects in the sky appear to be near one another.
  • Constellation: 1) A grouping of stars that appear to make a pattern as seen from Earth; 2) Officially, constellations are an internationally defined area of the celestial sphere.
  • Double [triple, quadruple, …] Star: 1) Two [or more] stars that are gravitationally linked together in orbit around each other; 2) Optical double stars are two stars at different distances to Earth that only appear to be very close together.
  • Elliptic: The path the Sun travels in the sky throughout the year.
  • Elongation: The angular separation as seen from Earth of two celestial bodies. For Mercury and Venus, it is the largest angle between the planet and the Sun.
  • Galaxy: A grouping of many millions or billions of gravitationally bound stars. Galaxies can be spirals or elliptical in shape.
  • Globular Cluster: A near spherical grouping of thousands of old stars.
  • Light-Year: The distance light travels in 31,556,952 seconds (one year)—about 5.8785 trillion miles.
  • Magnitude: A scale of perceived brightness, where large positive numbers are dim and large negative numbers are bright. The faintest stars to be seen by the unaided eye are +6.5 magnitude, the star Sirius is -1.4, the Full Moon is -12.74, and the Sun is -26.74.
  • Opposition: An event when a celestial object is opposite the Sun as seen from Earth. Or the Earth lies directly between the Sun and the object.
  • Variable Star: A star that changes brightness over time.

A Return Visit

It was July 4, 2005 when the Deep Impact probe encountered Temple 1, slamming a chunk of copper the size of a VW Beetle into the comet. The probe itself photographed the entire event. The results were a surprise. More dust and less ice were ejected from the impact site than expected; scientists compared the dust cloud to talcum powder. Observations also revealed that the comet was about 75 percent empty space. Unfortunately, the debris cloud was too thick for the probe to see the impact crater before it whizzed by the comet.

On February 15th, 2011, the Stardust spacecraft flew past Temple 1. Its closest approach was about 112 miles. Stardust was a spacecraft repurposed by NASA after a visit to the comet Wild 2 in 2004. Scientists wanted a second look at Temple 1. This was the first time that a comet was visited twice and provided an opportunity to observe the crater that was created by Deep Impact. Scientists also hoped to see changes caused by the comet’s latest close approach to the Sun. Stardust did find the 500-foot wide crater created by Deep Impact.

“We see a crater with a small mound in the center, and it appears that some of the ejecta went up and came right back down,” said Pete Schultz of Brown University in Providence, Rhode Island. “This tells us this cometary nucleus is fragile and weak based on how subdued the crater is we see today.”

The ride inside the coma of the comet was not smooth for Stardust. Telemetry received after the closest approach indicates the spacecraft flew through waves of disintegrating cometary particles, including a dozen impacts that penetrated more than one layer of its protective shielding.

“The data indicate Stardust went through something similar to a B-17 bomber flying through flak in World War II,” said Don Brownlee, Stardust-NExT coinvestigator from the University of Washington in Seattle. “Instead of having a little stream of uniform particles coming out, they apparently came out in chunks and crumbled.”

It may be years before scientists are able to figure out Temple 1 after sifting through all the pictures.

The Planets and Moon

  • Mercury will be hanging near the Sun this month. Look quick in the western twilight low on the horizon for a glimpse. Mercury and Jupiter have a conjunction on the 15th. Mercury will be faint and two degrees to the lower right of Jupiter one hour after sunset.
  • Venus is the bright “morning star” in the east before sunrise. Look 60 to 30 minutes before sunrise on the 1st and 31st for a conjunction on Venus and the Moon. The 1st will be with a waxing crescent moon, and the 31st will be with a waning crescent moon.
  • Mars is in the glare of the Sun this month.
  • Jupiter is near the western horizon at sunset. Look quick, or you’ll miss this planet. One hour after sunset on the 6th, look for Jupiter six degrees to the upper right of the Moon.
  • Saturn will be rising in the east after sunset.
  • The Moon will be new on the 4th at 1:40 a.m. and full on the 19th at 12:10 a.m. On the 15th, the Moon also reaches perigee, its closest approach to Earth in its orbit. It will appear to be 14 percent larger that evening than at its apogee on April 12th. On the 25th, the Moon will reach its southern most place in its orbit, 24 degrees south of the equator.
  • Daylight savings starts on March 13th.
  • Spring begins on the 20th at 5:21 p.m.




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