The Brightest Planets in August's Night Sky: How to See them (and When)
By Joe Rao, Space.com Skywatching Columnist |
MORE
Four bright planets span the evening sky at dusk. Over in the west-southwest is dazzling Venus, which sets after the end of evening twilight. Higher up in the west-southwest stands Jupiter, while in the south-southeast is Saturn. Finally, low in the southeast is Mars which spends August slowly diminishing in splendor from its pinnacle of brilliance that it reached at the end of July. It will appear only half as bright at the end of the month, compared to the beginning of the month. During the final week of August, after Mars has set, Mercury appears low above the east-northeast horizon at the break of dawn.
In our schedule, remember that when measuring the angular separation between two celestial objects, your clenched fist held at arm's length measures roughly 10 degrees. Here, we present a schedule below which provides some of the best planet viewing times as well directing you as to where to look to see them.
Mercury, which reaches inferior conjunction with the sun on August 9, will remain hidden from view for most of August. After mid-month, the planet will enter the eastern pre-dawn sky and will begin an apparition that peaks at greatest western elongation (18 degrees from the sun) on August 26. On that date, the optimal time for seeing Mercury will be at about 5:30 a.m. local time. Due to the steep morning ecliptic and Mercury moving through its ascending node on August 28, this will be an excellent morning apparition for observers in the Northern Hemisphere, and a poor one for Southerner Hemisphere observers. The planet will decrease in visual brightness during this apparition. Viewed in a telescope, Mercury's apparent disk size will shrink and wax to a gibbous phase at month-end.
Mercury – rushes between Earth and sun (passing inferior conjunction) and later in the month it emerges into the dawn twilight. It should be visible without optical aid by the 22nd. On August 26th, it reaches greatest elongation, 18-degrees west of the sun. This is a rather good morning apparition, because the line from planet to sun forms a steep angle with our August morning horizon. It rises 1½ hours before sunrise.
During August, Venus will start to wrap up a long apparition in the western evening sky. The extremely bright planet will swing wider of the sun until August 17, when it will reach its greatest eastern elongation 46 degrees from the sun, and then it will begin to swing sunward again. The planet will spend the entire month among the stars of Virgo, and finish August only 1.25 degrees below that constellation's brightest star Spica. Meanwhile the combination of a lower evening ecliptic and Venus' orbital motion, will bring the planet lower in our sky, shortening our observing time. Venus will continue to brighten during August, starting the month at magnitude -4.31 and reaching magnitude -4.61 at month-end. At the same time, the planet's apparent disk size will increase as it moves toward Earth, and its illuminated phase will decrease from 57% to 40%. In the western sky on the evening of Tuesday, August 14, the young waxing crescent moon will take up a position 6 degrees above Venus, making a lovely wide field photo opportunity.
Venus – continues to outshine August's other evening planets by a huge margin, brightening from magnitude -4.3 to -4.6 over the course of the month. It reaches its greatest elongation from the sun (46-degrees) on August 17th and resembles a roughly half moon phase (49-percent illuminated) through a telescope. Three days earlier, on the evening of the 14th, a four-day old crescent moon is poised high above Venus after sunset.
Mars, after reaching opposition and closest approach to Earth at the end of July, will spend August moving through the stars of western Capricornus. In early august, Mars will be a bright reddish naked-eye object visible all night long. At month-end it will peak in the south during late evening and set at 3 a.m. For mid-northern latitude observers, Mars will reach a maximum elevation of only 20 degrees above the southern horizon. Observers farther south will see Mars higher in the sky and through less of Earth's distorting atmosphere. On August 28, the Red Planet will complete a retrograde loop and resume regular eastward motion through the distant stars. As Earth slowly increases our distance from Mars, the planet will decrease in brightness; from magnitude -2.75 on August 1st to magnitude 2.1 on August 31st. At the same time, Mars' apparent disk diameter will decrease from 24.3 arc-seconds to 20.9.
Mars – dominates the sky east of Saturn. Fresh from last month's opposition and close approach to Earth, is still very bright and fiery. But it fades noticeably during August, from magnitude -2.8 to -2.1, while its disk in a telescope it shrinks in apparent size by about 15-percent. Earth is fleeing ahead of Mars around the sun and is now leaving the red planet behind. The planet is highest when due south, which corresponds to around 12:30 a.m. at the beginning of the month; two hours earlier at the end. An even wider gibbous moon can be found on the 22nd sailing far above Mars.
Very bright Jupiter will remain easily observable during August in the western evening sky, moving eastward through the stars of western Libra. In early August it will set at around midnight. By month end, it will set 90 minutes earlier. The planet will pass 0.5 degrees above the bright double star Zubenelgenubi (Alpha Librae) around mid-month, and the pair of objects will appear together in the field of view of a telescope at low magnification. From time to time during August, the little round shadows cast by Jupiter's four Galilean moons will be visible as they cross the planet's disk, frequently two at a time. On the evenings of August 16 and 17, the waxing crescent moon will be positioned 7 degrees northwest and northeast of Jupiter respectively.
Jupiter – hangs modestly high in the southwest at dusk. On August 6th, the king of the planets blazes at magnitude -2.1. This is also the day Jupiter reaches east quadrature (90-percent east of the sun). For many weeks around then, the eastern edge of its disk is noticeably shadowed, and eclipses of its largest moons are more readily observable. Jupiter sets as early as 10:20 p.m. (daylight saving time) by the end of the month. On the evening of the 17th, take note of Jupiter situated a good distance to the lower right of the moon. It is also passing only 0.6° north of Zubenelgenubi, Alpha (α) Librae. This is Jupiter's third rendezvous with this 3rd magnitude double star in less than eight months, and the closest in this unusual triple conjunction series.
Saturn will be visible during August as a medium-bright (visual magnitude 0.2), yellowish object in the southern evening sky and slowly moving retrograde westward through the stars of northern Sagittarius - just to the east of the Milky Way. Saturn will reach its highest elevation above the southern horizon in the latter part of the evening. Because the ringed planet reached opposition just days after the June 21 solstice, it will remain relatively low in the southern sky for observers in mid-Northern latitudes this summer. The rings, which subtend an angular size of about 42 arc-seconds, continue to be well open because the planet's northern pole is tilted roughly sunward. On the evening of August 20, the waxing gibbous moon will sit less than 4 degrees to the right of Saturn and the pair of objects will easily fit within the field of binoculars. At the end of August, the Ringed Planet will move within 2 degrees of several spectacular deep sky objects, Messier objects 8, 20, and 21.
Saturn – can be easily located by going out in late twilight and looking south-southeast at the beginning of August, or due south around month's end. Saturn is the bright "star" roughly a third of the way up in the sky; the farther south you are the higher it will be. Later in the evening Saturn swings low to the southwest. Below Saturn is the Teapot in Sagittarius. The pot starts August upright during twilight, then gradually tilts as if pouring in the following hours and weeks. As darkness falls on the evening of the 20th, look for Saturn well to the lower left of a 75-percent illuminated gibbous moon.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's Lower Hudson Valley
