August and September Skies

Printer-friendly versionPrinter-friendly version

Nature Magazine, August-September 1944

In this two-month period there will be several interesting conjunctions among the planets – Mercury with Venus and with Jupiter; Venus with Jupiter and Mars, as well as with Mercury.

Mercury will be in the evening sky until it comes into inferior conjunction with the sun on September 6, when it passes to the morning sky. It will be farthest east of the sun in the evening sky on August 10, and farthest west of the sun in the morning sky on September 22. The evening elongation will not be a favorable one. The planet will be then farther away from the sun – 27 ½ degrees – than at any time during the year, but its path cuts the horizon at such a small angle at this time that Mercury will be only 9 degrees above the horizon at sunset and difficult to observe in the haze of the evening twilight. Its morning elongation on September 22 will be much more favorable, however, for it will rise then more than one and a half hours before the sun and will be 16 degrees above the horizon at sunrise. On the following day there will be a close conjunction of Mercury with Jupiter. At that time Mercury will be more brilliant than Vega or Arcturus, and Jupiter more than twice as brilliant as Mercury. At the time of conjunction the two planets will be so close that they will resemble a double star. Shortly before sunrise on September 23, although not as near as at the time of conjunction, they will still be a close and brilliant pair in the morning sky. This will probably be the most favorable opportunity of the year to identify and observe this elusive planet.

The conjunction of Mercury with Venus on August 26 will be difficult to observe. Venus is low in the west and not well placed for observation in August. Its superior brilliance may make it observable, even in the glare of twilight, close as it will be to the horizon. It will be more difficult to find Mercury although Venus will be an aid as the two planets must be close together at the time. This is not a close conjunction of the two planets since Mercury will be at the time 6 degrees south of Venus. On September 9 there will be a conjunction of Mercury with Jupiter, but the two planets are at the time so close to the sun that neither of them is visible.

The position of Venus in the evening sky will gradually improve during these two months. The planet is now in the gibbous phase as seen in the telescope, and is still on the far side of its orbit from the earth. It will set within about one hour of the sun in September. On September 9 there will be a conjunction of Venus and Mars, difficult to observe because both planets will not be not far above the western horizon. Mars is now far from the earth and gradually drawing in toward conjunction with the sun. It will not be easy to find it in the twilight, even with the aid of Venus, which will be about half a degree north of it at the time of conjunction. Mars is now less brilliant than a star of second magnitude. About the middle of September Mars and Venus will be only a few degrees northwest of Spica in Virgo.

Jupiter passes from the evening to the morning sky on August 31, when it is in conjunction with the sun. It will, therefore, be seen low in the West in the early part of August after sunset, and low in the East before sunrise in the latter part of September. At the end of that month it will rise about one and a half hours before the sun in Leo.

Saturn will be in Gemini during these two months. In August it rises several hours before the sun and is high in the eastern sky at sunrise. By the end of September it rises shortly before midnight and is near the meridian at sunrise.

The Perseids, one of the annual showers of meteors, give a maximum display on August 11-12 after midnight, although they may be seen in smaller number on any night in the earlier part of the month. They will have some interference from moonlight this year. This should not be strong enough to blot them out altogether, however. The moon will be at the last quarter on August 10, rising about midnight, and the constellation of Perseus – in which the radiant of the shower lies – may also be seen in the northeast about the same time. Bright moonlight reduces greatly the number of meteors observable. The Perseids are swift and bluish-white in color, and come from the general direction of the constellation whose name they bear. Thus one will easily recognize a member of the swarm.

The Milky Way is particularly interesting at this time of year, when its sharp division into two branches is very noticeable in the evening sky. This apparent division is due to the presence of dark nebulous matter lying along the Milky Way in this part of its path and not to a real cleft in it, as was formerly believed. In the constellations of Scorpio and Sagittarius are some of the densest clouds of the Milky Way, and among them many wisps and patches of dark nebulosity. The center of our great galactic system lies in Sagittarius. Our solar system is estimated to be at a distance of 32,000 light years from this center, and the period of revolution around it is about 225,000,000 years. It follows from this that the mass of our Galaxy, or Milky Way system of stars, is 170,000,000,000 times the mass of the sun. The Galaxy is flattened, or lens-shaped, with a diameter of 100,000 light years and a thickness of 10,000 light years. Scattered above and below the Galaxy, to a distance of about 50,000 light years from the central plane, are the globular star clusters and many individual variable and non-variable stars.

Many great galaxies, similar in general to our own galaxy, lie at distances measured in millions of light years from our own galactic system. They are the well-known spiral nebulae or "island universes." One of the nearest of these and one visible to the naked eye is the Great Andromeda Nebula. Its distance from the earth is about 900,000 light years. The Large and Small Magellanic Clouds are the nearest of the external galaxies. The Large Cloud is at a distance of 75,000 light years, and is 12,000 light years in diameter. The Small Cloud is at a distance of 85,000 light years, and is only 6000 light years in diameter. They have the appearance of detached portions of the Milky Way, from which they are far distant, in southern circumpolar regions. The Large Cloud, Nubecula Major, as it is also called, is in the constellation of Dorado, The Goldfish, and the Small Cloud, Nu-becula Minor, is in the constellation of Tucana, The Toucan. It has been estimated by Shapley that there are half a million stars brighter than the eighteenth magnitude in the Small Cloud. Neither of these galaxies can be seen north of about 20 degrees north.

Autumn will begin this year in the Northern Hemisphere, and spring in the Southern Hemisphere, on September 22 at 11:02 P.M., Eastern Standard Time, when the sun will cross the Equator going south.