June and July Heavens

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Nature Magazine, June-July 1948

The planet Uranus has a new satellite, discovered recently with the 82-inch reflector at the McDonald Observatory on Mount Locke, Texas. The tiny object was found on a photographic plate taken on February 15 of this year, and also on two plates taken on March 1. So faint is the satellite that it is not observable visually, even in the largest telescopes, but can he easily photographed with an exposure of two or three minutes.

The four previously known satellites of Uranus are Ariel and Umbriel, discovered by Lassell in 1851, and Titania and Oberon, discovered by Sir William Herschel in 1787. They are among the faintest and most difficult to observe of all satellites, to be classed in this respect with the six faintest satellites of Jupiter, all discovered since the beginning of this century, and all less than 100 miles in diameter, and Phoebe, probably about 200 miles in diameter, faintest of the nine satellites of Saturn. The four historic satellites of Jupiter, as they are called, are 10, Europa, Ganymede and Callisto, known also as Satellites I, II, III, IV, in the order of their distance outward from the planet. They are bodies comparable in size with our own moon; two of them, in fact, are about 200 miles greater in diameter than the planet Mercury, and the other two are nearly equal in size to the moon. These four satellites, discovered by Galileo in 1610, are easily visible in the smallest telescopes, and they are continually presenting phenomena of never-failing interest to the observer, such as eclipses in the shadow of the planet, disappearances behind its disk, and, later, reappearances, as well as transits of the satellites and their shadows across the disk of the planet. All of these phenomena are predicted in advance for the benefit of those who wish to observe them.

Satellite V, nearest to Jupiter of its eleven satellites, is a difficult object to observe because it is so close to the planet, only about 25,000 miles above its surface, and also because it is so small and faint, with a diameter of about 100 miles. The remarkable discovery of this very difficult object was made visually by Barnard at the Lick Observatory in 1892.

The largest satellite of Saturn, Titan, is several hundred miles greater in diameter than our own moon, and is easily visible in the smallest telescopes. Rhea, second largest in size, has a diameter of about 1100 miles. Mimas, nearest to the planet of the four satellites, is about 400 miles in diameter, and skirts around the edge of the outer ring only 30,000 miles beyond it in a period of less than a day. Enceladus, Tethys, and Dione come next in order, small bodies a few ,hundred miles in diameter; then Rhea and Titan; Hyperion revolves around the planet beyond Titan in a period of about 21 days. It is a small body, only 300 miles or so in diameter and is at a distance of more than 900,000 miles from the planet. Japetus, which lies next beyond Hyperion, is about 1000 miles in diameter and is about 2,200,000 miles from the planet. It takes more than 79 days to complete one revolution around Saturn. Phoebe, the outermost of the nine satellites of Saturn, smallest of all of them, takes 550 days to revolve once around the planet at a distance of 8,000,000 miles from it. It is noted for the fact that it backs around the planet, or "retrogrades," as it is called, in an east to west direction.

The satellites of Uranus revolve around the planet in periods that range from about 2 ½ days for Ariel, and an estimated 30 hours for the new satellite, to 13 days, 11 hours for Oberon. The distance of the new satellite from the planet is estimated to be about 0.64 that of Ariel, or 75,000 miles. Titania, the largest, is about 1000 miles in diameter. The newly discovered satellite is probably not more than a few hundred miles in diameter. It is of the 17th magnitude, which is also the magnitude of Satellite IX of Jupiter, and the faintest in magnitude of any known satellite in the solar system. An object of this magnitude is beyond the limit of visibility to the eye in even the greatest telescopes, and is observed only on photographic plates.

It is a peculiarity of the satellites of Uranus that they revolve around the planet in a plane that is tipped nearly perpendicularly to the ecliptic, and in that plane, which is also that of the planet's equator, they back around the planet, or retrograde, as do Phoebe and several of Jupiter's satellites.

Triton, the lone satellite of Neptune, is estimated to be nearly as large as the planet Mercury. It is at a distance of more than 200,000 miles from Neptune and takes 5 days 21 hours to revolve around it.

In actual size the two tiny satellites of Mars are the smallest of all the known satellites in the solar system. Phobos, the larger, is about ten miles in diameter and Deimos only five miles.

As the larger and more powerful telescopes explore more thoroughly the regions in the neighborhood of the planets it will not be surprising if additional small satellites will be detected, and the present number of twenty-nine, in all, still further increased.

Among the planets during these two months we find Mercury well placed for observation the first few days in June in the evening twilight. It will be in inferior conjunction with the sun on June 23, passing then to the morning sky. It will be at greatest western elongation on July 16, when it will be a little north of east at sunrise, and should be visible in the morning twilight a week or more before and after this date.

Venus will be in inferior conjunction with the sun on June 24. It will still be conspicuous in the western sky early in June, but will draw in rapidly toward the sun. It will pass to the morning sky on June 24, and will soon be visible in the morning twilight. By July 15 it will rise about two hours before the sun, and it will be at greatest brilliancy in the morning sky on July 31.

Mars will be in the western evening sky during these two months. Early in June it will be a few degrees southeast of Regulus and passes from Leo into Virgo during this period. By the end of July it will be low in the southwest at sunset, and in brightness it will be about equal to Spica, which will not be far distant. The two objects will be a strong contrast in color, as Mars is red and Spica a brilliant bluish-white. Jupiter will be in opposition to the sun on June 15, when it will be visible all night. It will be well placed for observation in the evening sky during these two months, passing from Sagittarius into Ophiuchus in the southern sky. Saturn remains not far from Regulus in Leo during this two-month interval, and forms a conspicuous group with Mars and this first magnitude star, although Mars is now moving rapidly away in the direction of Spica. Saturn will be the brightest of the three objects. Mars, although brighter than Regulus in the earlier part of June, is equal to it in brightness late in June and early in July, but later in July becomes noticeably inferior in brightness.

In June few brilliant stars appear in the early evening. Regulus in the west, Spica in the south, Arcturus near the meridian, Vega in the northeast are the only first-magnitude stars well in view. Jupiter rising in the east near sunset, and Saturn and Mars in the west, help to brighten the heavens, however. The Milky Way is poorly placed in June along the eastern horizon. The heavens become more brilliant in July, with red Antares, and many bright stars in Scorpio, appearing. The Milky Way is now visible in the eastern sky. Deneb in Cygnus and Altair in Aquila, in the path of the Milky Way, shine in the northeast.

On June 21, at 7:11 A.M., Eastern Standard Time, the sun will be at highest northern declination in the constellation of Gemini and sign of Cancer, and summer will begin then in the northern hemisphere, and winter in the southern hemisphere.