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Musician-astronomer William Herschel achieved fame in 1781 by becoming the first person in recorded history to discover a planet. He wanted to name it after his patron, King George III of England, but it became known instead as Uranus — which, no matter how you pronounce it, still causes people to wince or roll their eyes. In this International Year of Astronomy, celebrating the 400th anniversary of Galileo's introduction of the telescope to astronomy, it's fun to think about how Galileo himself almost became the first planet discoverer since antiquity. In December 1612 and January 1613 he observed Neptune — but he didn't realize it! Nobody else did, either, until 30 years ago. Here's what happened. In the March 1979 issue of Sky & Telescope, amateur astronomer Steven C. Albers presented a list of mutual occultations of planets from 1570 (when Venus passed in front of Jupiter on February 5th) to 2223 (when Mars will cross Jupiter's face on December 2nd). In his accompanying comments, he noted that Neptune had passed very close to Jupiter a couple of times between the invention of the telescope in 1608 and the planet's actual discovery in 1846. He mused, "It would be extremely interesting if Neptune had been mistaken for a...star before it was discovered." Planetary scientist Charles A. Kowal and science historian Stillman Drake decided to follow up Albers's suggestion by doing some detective work. In Nature for September 25, 1980, they reported finding two drawings in Galileo's notebooks, one dated December 28, 1612 (reproduced below), and another dated January 28, 1613, that include Neptune with the label "fixa" — fixed star! So it appears that Galileo was the first person to lay eyes on Neptune, though he obviously didn't recognize it as an undiscovered planet. Given all the other discoveries he made, I think we can forgive him for missing this one!
Have you ever seen Neptune in a telescope? It's not much to look at — a tiny dot, barely distinguishable from a faint star except for its aquamarine tint. Unless you have a computerized telecope, it can be a challenge to find Neptune amid the starry background. But on May 27th this year, just as it did in Galileo's time, Neptune will pass so close to Jupiter that both planets will be visible in a telescope's eyepiece simultaneously. This makes it a snap to find Neptune — just point your telescope at brilliant Jupiter, which in late May is climbing the southeastern sky before dawn. The following image shows a 1° eyepiece field at 4 a.m. Eastern Daylight Time on May 27th, with celestial north up and east to the left (in many telescopes, the view will be oriented differently). Jupiter is accompanied by its four Galilean satellites: Callisto (C), Io (I), Europa (E), and Ganymede (G). To the west is the naked-eye star Mu Capricorni. And there, just north of Jupiter and its moons, is dim little Neptune.
If you don't feel like getting up before the Sun on May 27th but still want to see Jupiter and Neptune in the same eyepiece field, you'll get two more chances during IYA2009: in early July and late December. By December Jupiter will be well placed in the evening sky, making it more convenient to observe. Don't miss this chance to have a "Galileo experience" — with the significant difference that, unlike Galileo, you'll know you're looking at the planet Neptune, not just another background star!
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