The sky in July and August

Natural History, July, 2005 by Joe Rao

Mercury, still near the much brighter Venus after their close encounter late last month, shines in the western evening twilight at magnitude 0.1 as July begins. Mercury is less than a degree to the left of Venus; the two planets can be found just above the west-northwestern horizon about forty minutes after sunset. During the following two weeks the pair slowly separates, as Mercury appears to pivot around and below Venus. By the evening of the 13th, Mercury lies three and a half degrees directly below Venus, but Mercury appears only about half as bright as it did at the start of the month, so I recommend looking through binoculars to find the planet. After midmonth Mercury fades rapidly from the evening sky.

On August 5 Mercury reaches inferior conjunction, passing between Earth and the Sun. Thereafter the planet rushes up for a good dawn apparition during the latter half of the month. By the 17th Mercury has brightened to first magnitude and rises seventy-five minutes before sunup. On the 23rd the planet reaches its greatest western elongation, eighteen degrees from the Sun. At that point Mercury rises ninety minutes before the Sun and has brightened further, to magnitude zero.

Venus has been playing coy for a couple of months now, but the planet consents to showing itself ever so slightly longer in the twilight, as the summer progresses. Although always bright, the planet remains low in the sky all summer, as seen from midnorthern latitudes. On July 22 binoculars will help show Venus's close pairing with the star Regulus, the little king, in the constellation Leo, the lion. The planet and star are just a bit more than one degree apart, though the planet is much the brighter of the pair.

Venus's apparition in August is scarcely changed from what it was in June and July. The planet does make an approach to Jupiter.

Mars continues to approach Earth, and so it brightens dramatically in July and August. On July 1 the Red Planet, brilliant at magnitude -0.1, is exactly one astronomical unit--92.9 million miles--from Earth. Mars crosses the constellation Pisces, the fish, this month. On the 1st Mars rises nearly due east just before 1 A.M., and it rises about two and a half minutes earlier each night thereafter.

During August the distance from Earth to Mars shrinks to 62 million miles. Mars's brightness more than doubles as a result; it shines like a yellow-orange jewel, at magnitude-1; only one star, Sirius, shines brighter. Mars briefly passes through the constellation Cetus, the whale, from August 2 through the 5th, before moving into the constellation Aries, the ram, for the rest of the month. At the beginning of August Mars rises just before midnight in the east-northeast, and by the 31st it's rising before 10:30 P.M.

Jupiter is low in the west-southwest at dusk as July begins, and just gets lower (and sets earlier) as the month goes on. Jupiter remains in the constellation Virgo, the virgin, as it was in June, and the planet is moving eastward toward the constellation's brightest star, Spica, with each passing night; planet and star are separated by about fourteen degrees as July commences.

In early August Jupiter sets about an hour after dark, and just at the end of evening twilight as August ends. The planet continues moving eastward against the stars toward Spica; the gap between them closes to six degrees by the 31st. More notable--and noticeable--is the approach of Venus to Jupiter. On the 1st the two planets are separated by more than thirty degrees, but by the 31st they are within one and a half degree of each other.

As July commences, sky gazers might be able to see Saturn about forty minutes after sunset, just above the west-northwestern horizon; the planet is about seven degrees to the lower right of Mercury and Venus. Thereafter Saturn is swallowed up in the evening twilight.

The planet is not visible again until mid-August, when it appears in the east-northeast just before sunrise. A good time to look for Saturn comes on the morning of August 18, when it passes five degrees above Mercury.

In July the Moon wanes to new on the 6th at 8:02 A.M. It waxes to first quarter on the 14th at 11:20 A.M. and to full on the 21st at 7:00 A.M. Our satellite wanes to last quarter on the 27th at 11:19 P.M.

On the night of July 17th a waxing gibbous Moon occults, or hides, the bright reddish star Antares. The occultation is visible from parts of the far western and southern United States. Elsewhere, Antares seems to pass close to the Moon's upper limb. The best views come between about 8 P.M. and 9 P.M. on the 17th in the Pacific time zone, and between about 1 A.M. and 2 A.M. on the 18th in the Eastern time zone.

In August the Moon wanes to new on the 4th at 11:05 P.M. It waxes to first quarter on the 12th at 10:38 P.M. and to full on the 19th at 1:53 P.M. It wanes to last quarter on the 26th at 11:18 A.M.

The Earth reaches aphelion, its farthest point from the Sun, on July 5 at 1:00 A.M. The Sun is 94,512,036 miles away.

The Perseid meteor shower comes every August. This year it peaks during the predawn hours of the night of August 11th. The meteors are fast and bright, and they often leave persistent trails. From a location free from bright lights and tall objects, an observer might see some fifty to a hundred shooting stars an hour. The meteors are called Perseids because they appear to shoot away from the constellation Perseus, a hero of Greek myth. Because the Perseids usually make their display around August 10, which the Roman Catholic Church recognizes as the feast of Saint Lawrence, they have been referred to as the fiery "tears" of Saint Lawrence.