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| This month, Jupiter and Saturn appear
in the east-northeast. Saturn glows at mag 1.9 about 1 deg above the horizon.
Saturn is now about 8.3 AU from the earth, or about 770 million miles not
far at all in terms of the really great distances to the nearest galaxies
(as John Bell will tell us at our October meeting). Jupiter,
shining at mag -2.8, is about 5 deg above the horizon at 10:00 PM. Jupiter
is only about 4.3 AU from the earth at present, or about 400 million miles.
How long does it take Jupiter's light to get here?
The Andromeda Galaxy, about the size of our own Milky Way, is visible to the naked eye, mag 3.5, under good conditions as a pale patch about 57 deg above the ENE horizon. The galaxy subtends a visual angle of about 3 deg, which is 6 times the visual angle of the full moon! However, we see much less of the galaxy with our naked eyes because its surface brightness is very low. The magnitude of an extended source, such as a galaxy, is found in effect by squeezing all of its brightness into a star-point of light, so a mag 3.5 galaxy does not appear as bright as a mag 3.5 star (I owe this scholarly treatment of magnitudes to a discussion I once had with Dave LeVine, so if you have any quibble here, or any desire for more information on the topic, see Dave). The distance to Andromeda is about 3 million light years. Each light year is about 6 trillion miles, so if you want to know the distance in miles, do the arithmetic. By the way, unlike the really distant galaxies, Andromeda is approaching us at a velocity of about 21 miles per second. How long will it take to get here? Should we duck? The Double Cluster in Perseus, mag 4, is about halfway to Cassiopeia, about 40 deg above the horizon. These open clusters are visible to the naked eye in a good dark sky. These clusters of 300-400 stars are about 7,400 light years away, relatively close as astronomical distances go. Indeed, the diameter of our Milky Way galaxy is only about 100,000 light years, a respectable size as galaxies go, but not far at all in terms of the distances between galaxies. Humbling, isn't it? |
You can print a copy of this star chart so you can take it outside.
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