|4,878 km||3.30e23 kg||57,910,000 km (0.38 AU)||87.969|
Planet Mercury is the closest planet to the Sun and the eighth largest overall. It's surface resembles out moon, with lunar-type terrain. This is caused by the intense heat from the proximity of the Sun. Mercury's orbit is very eccentric, causing strange phenomenons if you were to observe its actions from the surface.
Mercury also has almost no atmosphere, causing eccentric weather conditions as well. Due to the proximity to the Sun, Mercury (during the day) can rise to over 400 degrees C (750 degrees F), but at night, because of the lack of atmosphere to help retain the heat, Mercury's temperature can drop to an unbelieveable -180 degrees C (-300 degrees F)!
Mercury is very dense (second densest major body in the Solar System). Mercury's interior is dominated by a large iron core (radius = 1,800 to 1,900 km), which is partailly molten, with its outer shell being just 500 to 600km thick. The mean surface temperature on Mercury is 179 degrees C.
Pluto was thought to be bigger than Mercury, because it is fairly bright, for a small planet way out there away from the sun. No one ever saw a visible disk, in any photo of Pluto. It was just a star-like dot on all photos. It was assumed to have a dark, rocky surface, like Mercury or the Moon. So, it's diameter was deduced from its brightness. Well, better photos have been made, and spectra of the surface have been taken. And it turns out to be very tiny indeed, with a very bright icy surface. It is about 1300 miles in diameter, roughly half the size of our moon.
So Mercury is now bigger than Pluto. But, how did astronomers make the mistake in Mercury's rotation period (length of its day)? There's a big difference between 88 days and 59 days.
Well, Mercury is very difficult to see. The sky is never dark when you can see Mercury, because Mercury is fairly close to the sun. In fact, before space flight, the best photos of Mercury have all been taken during the daytime, because that is the only time that Mercury is far above the horizon. There is too much atmospheric distortion near our horizon when Mercury is visible at dawn or dusk.
So, originally, Mercury's rotation period was deduced from theory. A rotation period that has synchronized itself to be the same as the orbital period is fairly common in the solar system. And, a planet that had been that close to the sun for the entire history of the solar system should have become synchronized by now. But, there are other forms of sychronization, and Mercury has stumbled onto a second kind. It's rotation period is 2/3 of it's orbital period. This seems to be a stable situation.
If Mercury's orbit were nearly circular, then this 2/3 ratio would not be stable. Mercury's spin would continue to slow down until the rotation period synchronized at 88 days. But, Mercury has a fairly eccentric orbit (noticeably elliptical). Because of this, Mercury's distance from the sun varies from about 29 million miles to about 43 million miles. That is quite a big difference. With the 58.7 day rotation period, Mercury's long axis is always lined up with the sun, when Mercury is closest to the sun (at perihelion). This is apparently enough synchronization to remain stable forever.
Data about Mercury:
Less than half of Mercury's surface has been mapped. Mercury has a weak magnetic field, which was not predicted, as it has a solid core, and rotates slowly.
Who told you Mercury doesn't rotate? It spins once around on its axis every 59 days! In 1889, astronomer G.V. Schiaparelli's observations of dark features on the planet's surface seemed to establish that its day was 88 days long, however in the early 1960's radar pulses reflected from the surface were successfully detected, and by 1965, G. Pettengill and R. Dyce were able to unambiguously determine that it rotated once every 59.3 =/- 2.0 days using Doppler radar techniques. This has been improved somewhat to the current 58.65 +/- 0.01 day period
Saturn and Venus will be in conjunction in early February in the early evening skies. Venus is that bright 'star' you see in the west during this time in early 1996. By February 1, they will be 1.3 degrees apart. This is worthy of a photograph using fast film and a telephoto lens at a few seconds exposure. Earth passes through the ring plane of Saturn in early 1996 so its rings will not be seen with a small telescope, making its identification as a planet only slightly tricky. It will still have a recognizable disk.
Mercury is an early morning object seen just before sunrise near the eastern point on the horizon where the Sun rises.
Mercury has been observed for thousands of years, and I believe the Babylonians are credited with mentioning it in writing for the first time some 3000 years ago or so. We do not know the name of the individual who first discovered it, just as we do not know the name of the man/woman who invented the wheel.
Because they are too close to the Sun. Any moon with too great a distance from these planets would be in an unstable orbit and be captured by the Sun. If they were too close to these planets they would be destroyed by tidal gravitational forces. The zones where moons around these planets could be stable over billions of years is probably so narrow that no body was ever captured into orbit, or created in situ when the planets were first being accreted.
Mercury's orbital speed changes throughout its orbit around the sun. For some locations on the surface of Mercury, when the rotation speed of Mercury drops below the speed of Mercury around the Sun, the Sun will rise and then after a few hours set, and then rise again to continue its motion across the sky. I do not know the dates for this phenomenon, but perhaps there are some planetaria simulators out there that would enable you to watch sunrise from Mercury's surface and by trial and error you could find the proper latitudes and times of the year when this happens. Sorry!