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Mercury , Venus, Mars Part I. Lecture 20. Observing Mercury and Venus from Earth. Elongations of Mercury

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Mercury, Venus, Mars

Part I

Lecture 20

observing mercury and venus from earth
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°.

maximum separation from the sun
Maximum separation from the Sun
  • Mercury’s greatest elongation : 18° to 28°
  • Venus’s greatest elongation : 47°
favorable and unfavorable elongations
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
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



mercury s rotation
Mercury’s Rotation

Synchronous rotation?

3 to 2 spin orbit coupling
3 to 2 spin-orbit coupling

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 s rotation
Venus’s Rotation

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!

mercury s surface
Mercury’s surface

As expected for a small planet,

Mercury’s surface has a lot of


a scarp 2km high and 600km long
A scarp : 2km high and 600km long

Cooling Mercury contracted and made a scarp…

mercury s interior
Mercury’s interior

Too large a core : 42% volume

One possibility

unexpected magnetic field in mercury
Unexpected magnetic field in Mercury

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.

speculations on venus in the pre spacecraft era
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
Speculations on Mars in the pre-spacecraft era

Ground-based image of Mars

  • William Herschel (late 1700s)
    • discoverer of Uranus
    • discovered polar ice caps of Mars
    • discovered the length of the Martian day

(24h 37 min)

    • noticed permanent marks on the surface
    • “… inhabitants probably enjoy a situation in many respects similar to our own!”
mirage of the martian canals
Mirage of the Martian Canals

Giovanni Schiaparelli (late 1800s)

discovered 79 linear features (“canali”)

meaning “channels”, but incorrectly translated as “canals”

Then, Percival Lowell…

percival lowell 1855 1916
Percival Lowell(1855-1916)

Built an observatory in 1894 at Flagstaff, AZ to observe Mars…

  • He knew that not all astronomers see Schiaparelli’s canals…
  • He thought that it is due to poor “seeing”

On top of Mauna Kea mountain

Due to the astronomical seeing effect, long exposed pictures get blurry.

lowell s interpretation of martian surface
Lowell’s interpretation of Martian surface
  • Mapped 200 canals. He knew that Mars is arid and its polar ice caps…
  • Canals  transportation of water from pole to equator cities…
  • One global network  a single nation.
  • But, many scientists did not believe his claim (w/o confirmation).
  • 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!
in summary
In summary…

Important Concepts

Important Terms


Spin-orbit coupling

Retrograde rotation

  • 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
  • Chapter/sections covered in this lecture : sections 11-1 through 11-4