Far-flung bodies traverse astronomically large orbits

Posted July 30, 2010, at 6:30 p.m.

A reader noted that Neptune is in nearly the same position today as when Johann Galle discovered it in 1846. She asked if a list of the planets with their years in terms of Earth years and their distance to the sun could be published. Here it is with the distances given in astronomical units, or AU. One AU is 93 million miles, the average distance of the Earth to the sun.

Year Distance (AU)

Mercury 0.24 0.39

Venus 0.62 0.72

Earth 1.00 1.00

Mars 1.88 1.52

Jupiter 11.86 5.20

Saturn 29.46 9.53

Uranus 84.01 19.20

Neptune 164.8 30.1

Pluto 248.5 39.5

Note that Pluto has not made a single orbit of the Sun since before the American Revolution.

Focus on the planets

Four planets all come into view on the western horizon at sundown as August opens. After midnight Jupiter rises in the east and travels toward the west as dawn approaches.

Mercury is very low in the west, to the lower right of Venus as the month opens. Faint to begin with, it fades steadily and will be gone by midmonth.

Venus dominates the western horizon at sunset and cannot be mistaken for any other star or planet.

Mars is so distant that details are not visible even with good telescopes. Look to the upper left of Venus for the faint red-orange point of light that is Mars.

Jupiter rises before midnight in the southeast and is up the rest of the night. The belts and zones of Jupiter are readily distinguishable, as are its four major moons, which will pass across the face of the planet and slip out of sight behind its disk on each orbit.

Saturn makes up the fourth member of the planets now congregating on the western horizon. Early in August Saturn is just to the upper left of Venus and to the upper right of Mars. The rings of Saturn are too edge-on to offer any good viewing.

Uranus rises with Jupiter and can be spotted as a blue-green disk three degrees to the west of the giant planet. The gap between the two will halve over the month.

Neptune is high in the southeast in the late evening hours where a good telescope and finder chart will be needed to spot the tiny blue-gray disk.

August events

1 Sunrise, 5:21 a.m. Sunset, 8:02 p.m. This is Lammas, or Loaf Mass Day, a cross-quarter day marking the midpoint between the summer solstice and autumnal equinox.

3 Moon in last quarter, 1 a.m.

8 Venus forms the base of a triangle in the west about an hour after sunset with Saturn to its upper right and Mars to its upper left respectively. Mercury is very low in the west about a half-hour after sunset and may be spotted with binoculars.

10 New moon, 11:08 p.m. The moon is at perigee, or nearest approach to the Earth, and this combination can give rise to abnormally high tides. The sun enters Leo on the ecliptic.

11 Mercury is just to the upper right of the crescent moon very low in the west just after sunset.

12 The Perseid meteor shower peaks tonight and absence of any interference from the moon should give good viewing. Look for about 60 meteors per hour out of the northeast from the direction of Perseus. The Perseids tend to be bright and often leave persistent trails.

16 Moon in first quarter, 2:14 p.m.

17 Mars is situated directly above Venus tonight.

23 The sun enters the astrological sign of Virgo but astronomically is still in Leo.

24 Full moon, 1:05 p.m. The full moon of August is known variously as the Sturgeon Moon, Grain Moon, Green Corn Moon, and the Dog’s Day Moon.

25 The moon is at apogee, or farthest distance from Earth.

31 Venus, Mars and the bright star Spica form a circle that makes up the closest “trio” of the year. Sunrise, 5:56 a.m. Sunset, 7:15 p.m.

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