Venus Is A Backwards Planet

This map shows Venus about 20 minutes after sunset this evening when it will shine ~4 (about three fingers) above the west-northwest horizon). It's joined by Mercury (use binoculars!) and Jupiter. Stellarium
This map shows Venus about 20 minutes after sunset this evening August 11 when it will shine  about 4° (three fingers) above the west-northwest horizon). It’s joined by Mercury (use binoculars!) and Jupiter. Stellarium

It’s fun to have Venus on the scene again. You can spot it at dusk very low in the northwestern sky starting about 20 minutes after sundown. Look for a solitary yellowish point of light struggling against the fading light of day. One fist to its upper left, you might see glimmering Mercury (it’s only magnitude +1) and further, a fist and a half from Venus, the departing spark of Jupiter.

Most planets spin counterclockwise, but not Venus and Uranus. Uranus isn't tilted nearly as much as that of Venus and rotates on its side. Venus's axis is tilted nearly 180°, so it spins backwards compared to the other six planets.
Most planets spin counterclockwise, but not Venus and Uranus. Uranus isn’t tilted nearly as much as that of Venus and rotates on its side. Venus’s axis is tilted nearly 180°, so it spins backwards compared to the other six planets.

The planet show is only a warm-up for the night’s big event, the Perseid meteor shower. For more details on times and how to view the shower, check my earlier blog on the topic. Today, we’re going to examine the remarkable fact that Venus rotates backwards. That means if you look down over the planet from far above its north pole, it would spin clockwise unlike most other planets in the solar system which spin counterclockwise. It also rotates v-e-r-y slowly, taking 243 days to turn just once on its axis. Since Venus goes round the sun every 224 days, a Venusian day is 19 days longer than its year! What’s going on here?

Venus orbits the sun every 224 days. From our perspective on Earth, it changes phase from full to half to crescent , then goes through the same sequence in reverse. Right now, it's in gibbous phase not far to the left of the superior conjunction point shown in the diagram.
Venus orbits the sun every 224 days. From our perspective on Earth, it changes phase from full to half to crescent , then goes through the same sequence in reverse. Right now, it’s in gibbous phase not far to the left of the superior conjunction point shown in the diagram.

Its ultra-long days may be the result of the sun’s powerful gravitation attraction which has slowed the planet’s spin down, similar to the situation on Mercury, which rotates only three times every two of its years. The closer to the sun a planet is, the stronger the solar tug. Earth rotates at 1,040 mph (1,674 kph) at its equator compared to Venus’s 4 mph (6.5 kph). If you walked briskly, you could easily keep up with Venus’s rotation! Further north or south of the equator, a lazy stroll would do.

One thing seems fairly certain: the planet’s rotation period and direction were different in the deep past. Astronomers hypothesize that gravitational nudging (called perturbations) by the early planets combined with the sun’s drag on Venus’s thick atmosphere caused unpredictable changes in the planet’s spin that could have flipped it a full 180.

One possibility for Venus's odd backwards rotation involves a massive impact by a protoplanet soon after the planet formed. Credit: NASA
Venus’s weird, backwards spin may have been caused by an impact from a protoplanet billions of years ago. Credit: NASA

An even more drastic scenario involves a massive impact. Our own moon likely resulted from material blasted into space when a Mars-sized planet sideswiped Earth 20-100 million years after the solar system formed. Venus may have experienced a similar impact but head-on.  The impactor, probably a protoplanet or large asteroid, transferred the energy of its motion and direction of movement (called angular momentum) to Venus, not only slowing it down but reversing the direction of it rotation. Who knows? Prior to the impact, Venus may have been slowly rotating in the normal way, making it easier for a massive impact to slam it into reverse.

The solar system is riddled with mysteries. We study the light of distant planets, take measurements, probe their atmospheres, cores and magnetic fields with orbiting satellites, and yet some things remain unproven for a long time until new information comes to light. In the meantime, science provides a set of plausible possibilities based on what we know up to this point. So it is with the origin of the stance and spin of Venus.