Mercury , Venus, Mars Part I. Lecture 20. Observing Mercury and Venus from Earth. 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 224.7-day period. The angle between Venus and the Sun at eastern or western elongation is 47°.
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
At least favorable
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).
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 116.8 Earth days.
And, Venus rotates in the opposite direction (clock-wise) retrograde rotation. Sun rises from the West on Venus!
As expected for a small planet,
Mercury’s surface has a lot of
Cooling Mercury contracted and made a scarp…
Too large a core : 42% volume
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.
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!
Ground-based image of Mars
(24h 37 min)
Giovanni Schiaparelli (late 1800s)
discovered 79 linear features (“canali”)
meaning “channels”, but incorrectly translated as “canals”
Then, Percival Lowell…
Built an observatory in 1894 at Flagstaff, AZ to observe Mars…
On top of Mauna Kea mountain
Due to the astronomical seeing effect, long exposed pictures get blurry.