Space/Science — HabitableZone

Space/Science » in reply to re: Couple of questions:

Did some digging. maybe this will help.
REB — (archived)

I found this.

From the article;

“As seen from Earth, Mercury is never far from the Sun. It is therefore observable only in twilight, when it is low in the sky but the sky is dark, or in daylight, when it is high but contrast is poor. Also, it's very tiny. Observers mapped a few faint markings on Mercury and concluded that because of solar tidal effects, Mercury always showed the same face to the Sun. Below is one map of the surface markings on Mercury as mapped from Earth.

“This misconception was heightened by a double numerical coincidence: Mercury and Earth line up every 116 days, almost exactly two rotations of Mercury. Mercury is highest in the evening sky in winter and in the dawn sky in summer for Northern Hemisphere observers, and three alignments of Mercury and the Earth take 348 days, very nearly a year. Thus, an astronomer observing Mercury at its most favorable would see it for years on end at the same time of year and always showing the same face to the sun! An observer who spent years seeing the same predictable markings on Mercury would be justified in concluding it always showed the same face to the Sun.”

And if you trust Wikipedia.

“Until radar observations in 1965 proved otherwise, it was thought that Mercury was tidally locked with the Sun. Instead, it turned out that Mercury has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbit makes this resonance stable. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it were totally locked.”

And they said this about Venus;

“A curious aspect of Venus' orbit and rotation periods is that the 583.92-day interval between successive close approaches to the Earth is almost exactly equal to 5 Venusian solar days (precisely, 5.001444 of these), making approximately the same face visible from Earth at each close approach. Whether this relationship arose by chance or is the result of some kind of tidal locking with the Earth is unknown .”

Log in or register to post.

The whole thread (70 posts)