Galileo and 400 Years of Telescopic Astronomy by Peter Grego & David Mannion

Author:Peter Grego & David Mannion
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Future transits of Mercury

Date

Time (UT, mid-transit)

Duration

2016 May 09

14:57

07 h 30 m

2019 Nov 11

15:20

05 h 29 m

2032 Nov 13

08:58

04 h 26 m

The transit of Mercury on May 7, 2003, observed (Credit: Peter Grego)

It was to be quite a while before telescopes became powerful enough for astronomers to discern markings on the tiny disk of the innermost planet. Subtle Mercurian surface features were recorded between 1780 and 1815 by the great lunar and planetary observer Johann Schröter (1745–1816), who mainly used the 16.5-cm reflector (the mirror of which was made by Herschel) at his observatory at Lilienthal near Bremen in Germany. He consistently observed that the phase of Mercury always appeared more concave than predicted; at predicted dichotomy, when the planet ought to have been a neat half-phase, Mercury seemed slightly crescent shaped. This phase anomaly is caused by the roughness of Mercury’s surface and the gradual dimming of light near the terminator; the phenomenon remains observable.

Schröter also noted that the southern horn of the crescent Mercury often appeared somewhat blunted, leading him to speculate that this was caused by the shadow of a huge mountain some 20 km high. Interestingly, the great observer William Herschel found it difficult to discern anything at all on the Mercurian disk, and he discredited Schröter’s claim that Mercury had an atmosphere.

Based on his observations, Schröter was of the opinion that Mercury’s day was 24 h and 4 min long; Friedrich Bessel used the same observations to arrive at what he thought was an accurate figure of 24 h 53 s and calculated that the axis of Mercury was inclined by a staggering 70° to its orbit around the Sun.

Observations of Mercury by Schröter

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