Mercury: A Difficult Observing Target

1. Mercury: A Difficult Observing Target • Nearest planet to the Sun. • Never more than 28 degrees away from the Sun. • Legend says that Copernicus lamented that he had never seen Mercury himself.

2. Alignments of Mercury

3. Mercury Transits • Transits provide very accurate measurements of orbital parameters. • Don’t occur at every inferior conjunction: so relatively rare (something like 7 years apart). • Example: Last three occurred in 1986, 1993, and 1999.

5. Le Verrier’s Results • Produced the first theory of Mercury’s orbit in 1843. Not perfect, since it was based mostly on older observations. • Reworked theory using most recent and accurate observations, resulting in a revised theory, published in 1859, including the discovery of an anomalous movement.

6. Mercury’s Orbital Precession; each ellipse is slightly different than the previous one

7. Possible Causes? Le Verrier’s proposed solutions: Two possibilities: • Errors in the estimated masses of the planets, particularly Venus. • An as yet undiscovered planet: Vulcan!

8. Vulcan • Le Verrier proposed either a single planet, several small planets, or even a disk of material. • Numerous false identifications, no definitive discovery. • Lots of people were looking

9. Simon Newcomb • 1882: American astronomer Newcomb verified Le Verrier’s theory of the motion of Mercury. • 1895: Produced detailed work on the orbits of Mercury, Venus, Earth, and Mars, which became the basis for the tables of these planets’ motions in the main national ephemeris.

10. Simon Newcomb • His solution: An alteration of Newton’s law of gravitation. The change made the tables very accurate, but was abandoned within just a few years as no one could believe this (still relevant today with respect to Dark Matter)

11. Finally, Einstein • General Relativity: 1915. • Einstein used Le Verrier’s anomalous advance of Mercury as a prime application of his theory: it gave the correct answer! Space Time is Curved by Mass