Check It Out: Planetary Triangle Forming in the Evening Sky
By Geoff Gaherty
Starry Night Education
posted: 28 July 2010
10:00 am ET
A trio of planets converging in the night sky this week and for several nights will give casual skywatchers the perfect chance to easily see and identify worlds they might not normally notice. The event, building up to a super celestial snuggle in early August, is also a chance to watch and grasp orbital mechanics in action.
Venus, Mars and Saturn will gradually, night after night, move into a tight triangular grouping in the early evening sky. (This graphic shows where to look to spot the planetary triangle on Aug. 5.)
Venus is so bright, you can't miss it, and that will allow you to locate the other two worlds with no trouble. You can start watching the spectacle tonight. [Venus photos from around the world.]
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AboutAstro.com/horoscopeThe ancients believed planetary alignments to be full of mystery portending dreadful events. Now we realize that, because the planets all move in a common plane called the ecliptic, they regularly line up with each other over time as seen from planet Earth.
The ecliptic is the same path across the sky followed by the sun, and so the three planets will appear to be following the sun into the Western horizon as darkness falls.
The ecliptic corresponds to the plane in space that Earth and the other planets travel in as the orbit the sun. Since the inner planets, like Venus, make their annual orbits faster than the outer worlds, like Mars, they're constantly trading places in our sky, converging and receding from one another as seen from our vantage point.
Planetary alignments are known to astronomers as conjunctions or appulses. The moon, which is also traveling close to the plane the planets orbit in, is in conjunction with each of the planets at some point every month. Conjunctions between pairs of planets, particularly the faster moving inner planets, occur several times a year.
Conjunctions of three or more planets are much rarer. This one has been building for several months, and will reach its culmination in the coming week.
What you'll see
Look in the western sky just after sunset any night this week. Your eyes will immediately be drawn to the brilliant planet Venus – it's brighter than any night sky object except the moon.
Look more closely as the sky gets darker, and you will see that Venus is accompanied by two lesser lights. These are the planets Mars and Saturn.
On Saturday night, July 31, Mars will be just below Saturn, less than two degrees away (your fist on an outstretched arm measures about 10 degrees of the sky). The following Saturday, Aug. 7, Venus will have moved westward so that it is less than three degrees below Saturn.
All three planets are moving eastward (left to right for viewers in the Northern Hemisphere) against the background stars of the constellation Virgo. The solar system's inner planets move across our sky (and around the sun) in much less time much than the outer planets, and so they rapidly move past distant Saturn.
On Thursday night, Aug. 5, the three planets will be closest together, forming a tight triangle that will easily fit in the field of binoculars.
Appearances are deceptive. Although close together in the sky, the three planets are actually at very different distances from the Earth.
Venus is closest at 0.796 astronomical units distance, Mars next at 2.022 astronomical units, and Saturn a distant 10.191 astronomical units. Because distances to other planets are, well, astronomical if measured in miles or kilometers, astronomers usually measure them in astronomical units: an astronomical unit is the average distance between the Earth and the Sun, 93 million miles or 150 million kilometers.
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