TSTW 1/28/10 (for Dodgeville, Wisconsin)
Thursday, January 28
Fabian Gottlieb von Bellingshausen (1778-1852), Russian naval officer and explorer, and the crews of his two ships, the Vostok and the Mirnyi, discovered Antarctica, 190 years ago (1820).
The Cassini Saturn orbiter will pass 4,654 miles from the surface of Titan at 4:29 p.m.
The Iridium 56 satellite will flare to -6 magnitude around 6:00 p.m. at azimuth 36° (NE) and altitude 61°, in Camelopardalis, during nautical twilight.
The 14.0-magnitude asteroid 283 Emma may pass in front of the 11.8-magnitude star Tycho 1220-449-1 in Aries for up to 8.0 seconds around 8:36 p.m. For more info, visit http://asteroidoccultation.com/asteroid.htm.
Friday, January 29
Mars is near the Moon this evening; they are closest at 9:15 p.m.
Saturday, January 30
The Moon is closest to SW Wisconsin this year at 12:11 a.m. (216,979 miles, surface-to-surface).
Full Moon, 12:18 a.m.; rises 4:46 p.m. Friday; transits 12:11 a.m.; sets 7:22 a.m.; = +16°
Cleostratus crater and the NNW limb of the Moon (that's NNE on the sky) are favorably tilted towards the Earth due to libration this morning before dawn.
The Lacrosse 4 rocket body will move from SSW to NE from about 6:04 to 6:12 p.m. It reaches its highest point in the sky (67° altitude, magnitude 2.1) around 6:08 p.m., between the Pleiades and Auriga, during nautical twilight.
Sunday, January 31
Cusanus crater and the NNE limb of the Moon (that's NNW on the sky) are favorably tilted towards the Earth due to libration this morning before dawn.
Look for the zodiacal light in the west beginning around 6:49 p.m., tonight through February 14.
Monday, February 1
The Moon crosses the celestial equator heading south.
Tuesday, February 2
Saturn is (sort of) near the Moon this evening.
The Progress 36P robotic cargo ship is scheduled to be launched at 9:45 p.m. from the Baikonur Cosmodrome, Kazakhstan, on a mission to the International Space Station.
Wednesday, February 3
M53, globular cluster in Coma Berenices, was discovered by Johann Elert Bode (1747-1826), 235 years ago (1775).
Thursday, February 4
Spica is near the Moon this morning before dawn.
The most accurate way to measure the distance to the nearer stars is to accurately measure their angular positions relative to much more distant stars twice each year, six months apart. The baseline of the Earth's orbit (2 AU) and the angular shift between the two measurements allows you to calculate the distance to the star using trigonometric parallax--the rock-solid foundation of all cosmic distance measurements.
Using ground-based telescopes, you would image stars about two hours west of your celestial meridian at the end of evening twilight and six months later those same stars--now two hours east of your celestial meridian--as morning twilight begins, night after night, throughout the year. Stars you observe now in the evening you'll be observing again in the morning in six months, and vice versa.
Ground-based parallax measurements through our messy atmosphere limit the stellar distances we can measure to just a few hundred light years, because the farther away a star is, the smaller is its annual parallactic angle. The Hipparcos satellite extended this out to about 1,600 ly, and the Gaia satellite to be launched in 2012 will accurately measure tiny parallax angles for stars out to tens of thousands of light years.
If you go to Simbad, you'll find for Rigel a listed parallax of 4.22 ± 0.81 mas (milliarcseconds, 10-3 arcseconds, which is 2.78 x 10-7 degrees). What this means is that if you observed Rigel the evening of March 13, 2010 and again the morning of September 11, 2010 (six months later), you'd measure a shift of 8.44 mas against the distant background stars--twice the parallactic angle since your baseline is 2 AU. By convention, the trigonometric parallax is the angle subtended by the radius of the Earth's orbit at the distance of the star.
We use simple trigonometry to calculate the actual distance to the star, so skipping over a couple of steps we get
Again, by convention, the parallax is denoted using the symbol (though this has nothing to do with the number ), and this is 1/2 the shift you would measure looking at the star six months apart. The mas subscript denotes milliarcseconds. The distance is in light years. Now you know how to use any Simbad parallax to calculate the most accurately known distance to that star in light years!
Now, let's say we launched a pair of parallax satellites into the Earth's orbit around the Sun, one leading the Earth by 90°, and one trailing the Earth by 90°. This would allow you to instantaneously measure trigonometric parallax by radioing the measurements from both satellites back to Earth simultaneously for analysis. Since the orbital period of the two satellites would be one year, they would be able to cover the entire sky in a year.
We could double the accuracy of our parallax measurements (or extend our range out twice as far), by putting two satellites in orbit 2 AU from the Sun, a bit further out than the orbit of Mars. This would give us a baseline of 4 AU, and now we would measure a shift for Rigel of 16.88 mas. We would divide by 4 to get the parallax in milliarcseconds, but our number would be twice as accurate as the angle we are measuring is twice as big. However, being further from the Sun, it would now take 2.8 years for our satellites to cover the entire sky.
If we put a pair of satellites 180° apart into an orbit 10 AU from the Sun--a little beyond the orbit of Saturn--we would improve the accuracy of our distance measurements (all else being equal) by an order of magnitude -- 10 times, or measure the distance to stars ten times as far away as we can from a 1 AU orbit. Again, looking at Rigel, our intrepid robotic explorers would simultaneously measure a total shift of Rigel against the much more distant background stars of 84.4 milliarcseconds. But it would take these two satellites 31.6 years to cover the entire sky in that distant orbit.
And so it goes!
The Soviet robotic lunar explorer Luna 24--the last spacecraft to soft land on the Moon--touched down in Mare Crisium (Sea of Crises) on August 18, 1976. That same day, Luna 24 drilled down 1.6 meters beneath the lunar surface, and later returned a 170 gram core safely to Earth. There, scientists found a layered structure, laid down by successive deposits of lunar material.
The Luna 24 core is the deepest sample of lunar material ever investigated--no further down than a 5' 2" person is tall.
