Herschel gives Mars seasons, and inhabitants
Herschel put the tilt of Mars near 29 degrees, read the shrinking polar caps as seasons, and concluded its inhabitants probably lived much as we do.
Between 1777 and 1783, William Herschel followed the white patches at the poles of Mars through four oppositions — beginning as a jobbing musician in Bath who ground his own mirrors, finishing as the king's astronomer at Datchet on a salary from George III. In 1784 the Royal Society printed the result under a title that gives away the whole argument: On the remarkable appearances at the polar regions of the planet Mars, the inclination of its axis, the position of its poles, and its spheroidical figure.
The caps behaved reciprocally. When the southern patch was large the northern was small, and over months each in turn shrank and regrew. From the way the markings wheeled across the disc, Herschel worked out the tilt of the rotation axis and put it at 28 degrees 42 minutes; the true figure is 25.19 degrees, so he ran about three and a half degrees high. He timed the rotation at 24 hours 39 minutes 21.67 seconds, some two minutes long against the modern sidereal value. He measured the disc as slightly flattened at the poles, and noted bright belts appearing and fading that he could ascribe to nothing but clouds and vapours in an atmosphere.
He was not starting from nothing. Giacomo Maraldi, at the Paris Observatory, had already shown at the oppositions of 1704 and 1719 that the southern cap wobbled as the planet turned — so it was not centred on the pole of rotation — and that it could shrink almost to vanishing and come back. Maraldi suggested ice. Herschel supplied the mechanism: an axial tilt close to Earth's, producing seasons, driving snow and ice on and off the poles.
That much has held up for 240 years. The sentence that ended the paper has not. Mars, Herschel concluded, "has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to our own."
There are no inhabitants, and the atmosphere is neither considerable nor moderate. Surface pressure on Mars averages about 6 millibars, under one per cent of sea level on Earth, and it is roughly 95 per cent carbon dioxide. The caps are a mix of water ice and frozen carbon dioxide, and the carbon dioxide part of them sublimes straight into the air each spring and freezes back out each autumn. Nothing there enjoys a situation similar to ours.
The trouble is that Herschel's reasoning was good. Same day length, same tilt, visible weather, seasonal ice — every analogy he had pointed the same way, and the instruments of 1784 could not supply the one measurement that would have stopped him. His confident closing line went into the literature alongside the correct numbers, and for the next century and a half it was the half that people remembered.