Observing Mercury and Venus from Earth Elongations of Mercury Elongations of Mercury Mercury moves around the Sun every 88 days in a rather eccentric orbit. As seen from the Earth, the angle between Mercury and the Sun at greatest eastern or western elongation can be as large as 28° (when Mercury is near aphelion) or as small as 18° (near perihelion). By contrast, Venus follows a larger, nearly circular orbit with a day period. The angle between Venus and the Sun at eastern or western elongation is 47°. Mercury moves around the Sun every 88 days in a rather eccentric orbit. As seen from the Earth, the angle between Mercury and the Sun at greatest eastern or western elongation can be as large as 28° (when Mercury is near aphelion) or as small as 18° (near perihelion). By contrast, Venus follows a larger, nearly circular orbit with a day period. The angle between Venus and the Sun at eastern or western elongation is 47°.

Maximum separation from the Sun Mercury’s greatest elongation : 18° to 28° Mercury’s greatest elongation : 18° to 28° Venus’s greatest elongation : 47° Venus’s greatest elongation : 47°

Favorable and unfavorable elongations Not only the separation, but also the combined effect of the tilt of Earth’s axis, the inclination and eccentricity of Mercury’s orbit play a role

Favorable and unfavorable oppositions of Mars Mars can be 0.37AU away at the most favorable opposition  can be 3.5 times brighter than Sirius! At least favorable opposition: 0.67AU

Mercury’s Rotation Synchronous rotation?

Due to the elliptical orbit of the Mercury (can never force the elongation always pointing to the Sun), Mercury’s long axis point to the Sun only when it is nearest to the Sun (i.e., at perihelion). Mercury spins 1.5 times while it make one orbit (1.5 to 1 spin-orbit coupling or 3:2). 3 to 2 spin-orbit coupling

Venus is always covered with thick clouds and have to see through this thick cloud  radio and IR radar technique. Venus’s rotation period (solar day) is Earth days. And, Venus rotates in the opposite direction (clock-wise)  retrograde rotation.  Sun rises from the West on Venus! Venus’s Rotation

Mercury’s surface As expected for a small planet, Mercury’s surface has a lot of craters…

A scarp : 2km high and 600km long Cooling Mercury contracted and made a scarp…

Mercury’s interior Too large a core : 42% volume One possibility

Mercury has a weak magnetic field (1% strength of that of the Earth).  unexpected for a small planet covered with lots of craters.  small size and slow rotation (58.6 days) make the existing magnetic field a puzzle. Unexpected magnetic field in Mercury

Speculations on Venus in the pre-spacecraft era Tropical Paradise? With no high concentration of CO2 in the Venusian atmosphere, it would have achieved a hot summer tropical weather (45°C). But, Mariner 2 found out that the Venus should be at least 400°C hot and later Venera 7 (descended to the ground of Venus) measured a temperature of 460°C!

Speculations on Mars in the pre-spacecraft era William Herschel (late 1700s) William Herschel (late 1700s) discoverer of Uranus discoverer of Uranus discovered polar ice caps of Mars discovered polar ice caps of Mars discovered the length of the Martian day discovered the length of the Martian day (24h 37 min) noticed permanent marks on the surface noticed permanent marks on the surface  “… inhabitants probably enjoy a situation in many respects similar to our own!” Ground-based image of Mars

Giovanni Schiaparelli (late 1800s) Giovanni Schiaparelli (late 1800s) discovered 79 linear features (“canali”) meaning “channels”, but incorrectly translated as “canals” Then, Percival Lowell… Then, Percival Lowell… Mirage of the Martian Canals

Percival Lowell ( ) He knew that not all astronomers see Schiaparelli’s canals… He knew that not all astronomers see Schiaparelli’s canals… He thought that it is due to poor “seeing” He thought that it is due to poor “seeing” On top of Mauna Kea mountain Built an observatory in 1894 at Flagstaff, AZ to observe Mars… Due to the astronomical seeing effect, long exposed pictures get blurry.

Lowell’s interpretation of Martian surface Mapped 200 canals. He knew that Mars is arid and its polar ice caps… Mapped 200 canals. He knew that Mars is arid and its polar ice caps… Canals  transportation of water from pole to equator cities… Canals  transportation of water from pole to equator cities… One global network  a single nation. One global network  a single nation. But, many scientists did not believe his claim (w/o confirmation). But, many scientists did not believe his claim (w/o confirmation). Excepting a couple of large geometrical features, his canals were purely imaginary! Excepting a couple of large geometrical features, his canals were purely imaginary! All scientists abandoned Martian canals (by 1910), the idea persisted among public for a long long time! All scientists abandoned Martian canals (by 1910), the idea persisted among public for a long long time!

In truth, Mars is a windblown, cratered world

In summary… Important Concepts Favorable and unfavorable configurations of Mercury, Venus, and Mars Mercury’s 3:2 spin-orbit coupling Venus’s retrograde rotation Surface features of Mercury Large iron core of Mercury Surprising magnetic field in Mercury Fantasies on Venus/Mars during pre-space era Important Terms Elongation Spin-orbit coupling Retrograde rotation Chapter/sections covered in this lecture : sections 11-1 through 11-4