The sky in July and August

Natural History, July-August, 2006 by Joe Rao

Mercury is too dim and too close to the Sun to see until the last few days of July. It passes through inferior conjunction, between the Earth and the Sun, on July 18 and moves into the morning sky. Look for it at the end of the month, low in the east-northeast sky about forty-five minutes before sunrise.

In August this speedy planet makes a fine morning apparition, as it gains altitude and reaches its greatest western elongation on August 7. On that morning it shines at magnitude 0.1 and rises nearly ninety minutes before sunup. During the next two weeks Mercury slowly drops back toward the Sun, but as if to compensate, it grows progressively brighter. On the mornings of the 9th and 10th you'll find Mercury roughly two degrees below brilliant Venus. Then, on the morning of the 20th, Mercury shifts to just a little more than one degree above Saturn. Two mornings later, a slender crescent Moon, less than thirty-six hours from its new phase, appears to hover well above Mercury. By then, Mercury brightens to magnitude -1.4, matching Sirius, the brightest star in the sky. For the rest of the month it disappears into the dawn.

Venus shines brilliantly at magnitude -3.8 in the morning sky. From the beginning of July through the middle of August it rises out of the east-northeast sky just as dawn breaks. In August it passes a couple of degrees above Mercury on the mornings of the 9th and 10th. In the second half of August it loses altitude noticeably as it begins its plunge back toward the Sun. A narrowing crescent Moon slides past Venus on the mornings of the 21st and 22nd. By the end of August Venus is rising a bit more than an hour before the Sun.

Mars starts July setting a little more than two hours after the Sun. But the planet is shining at only magnitude 1.8--as dim as it can get. Look for it low in the west-northwest sky, beginning about an hour after sunset. Don't confuse it with Saturn, which is about three times brighter, but well below and to the right of Mars. If you are blessed with very clear weather oil the evenings of the 21st and 22nd, look for Mars hovering less than a degree above and to the right of Regulus, the brightest star in the constellation Leo, the lion. You'll probably need binoculars to pick them up, but if you do, you'll likely be impressed by the color contrast between yellow-orange Mars and bluish Regulus.

This summer Jupiter is the most favorably placed planet to view. At the start of July it's shining brightly at magnitude -2.3 in the south-southwest sky and doesn't set until around 2 A.M. local daylight time. At the start of August it's in the southwest at sunset and sets around midnight. By the end of August it's setting about two and a half hours after sunset. Jupiter's retrograde, or westward, motion among the stars ends on July 6; thereafter, it starts shifting back to the east and will approach the star Zubenelgenubi, also known as Alpha Librae in the constellation Libra, the scales, for the rest of July and August. By August 1, the star and the planet are within five degrees of each other, but the two bodies continue closing in to a separation of less than two degrees by month's end. Look for Jupiter at dusk on the 29th, hovering above and to the right of a fat crescent Moon.

Saturn, for the first couple of weeks of July, can be spied low in the west-northwest sky for about an hour after sunset. Ultimately, however, the planet disappears into the glow of evening twilight. Use Mercury and brilliant Venus to guide you to Saturn as it emerges into the morning sky in late August. On the 20th, Saturn approaches to within slightly more than one degree of Mercury; both planets lie below and to the left of Venus. A sliver of a crescent Moon passes well above and to the left of Saturn on the morning of the 22nd. The next morning, Mercury and Venus form the end points of a diagonal line about eight degrees long against the sky, while Saturn lies almost exactly halfway in between. By the 26th, Saturn lies a half degree below and slightly to the left of Venus. By the following morning it is a half degree above and slightly to the right of Venus. But compared with that dazzling morning "star," Saturn appears only a forty-eighth as bright.

The Moon in July waxes to first quarter on the 3rd at 12:37 P.M., and becomes full on the 10th at 11:02 P.M. It wanes to last quarter on the 17th at 3:12 P.M. and to new on the 25th at 12:31 A.M. On the 20th a waning crescent Moon, 24 percent illuminated, occults, or passes in front of, the Pleiades star cluster as seen from eastern North America. That early-morning event takes place about a quarter of the way up in the eastern sky. In some ways it will be the reverse of the occultation of April 1. This time, between roughly 3 and 5 A.M. eastern daylight time, stars disappear along the Moon's sunlit crescent and reappear from behind the dim earthlit edge.

In August the Moon waxes to first quarter on the 2nd at 4:46 A.M., and to full on the 9th at 6:54 A.M. The Moon wanes to last quarter on the 15th at 9:51 P.M., and returns to new on the 23rd at 3:10 P.M. The Moon then comes back to first quarter for the second time in August on the 31st at 6:56 P.M.