Chapter 8After revisiting refraction – and showing no more understanding of it than in chapter 5 – the author fully considers diurnal parallax, as used to measure the distance to Mars in 1877 and thence calculate the parallax of the Sun.
The author seems to think Gill the astronomer only measured angles from Mars to a star in the evening and morning on Ascension Island and thereby calculated his result. The author repeats his confused contention that no angle could be measured because of his muddled ideas about astronomical theory. He is again mistaken in a number of important ways.
Back in chapter 4, Hickson made the important point, repeated here, that in the time between evening and morning observations, Earth is presumed to be moving through space in its orbit, as is Mars, so angles from a presumed baseline are changing in that time. He illustrates this with diagram 20 (page 46) showing his presumed scenario: –
Notice how Mars (M) has moved further in its orbit than Earth, in accordance with Hickson’s argument, but he previously illustrated this same problem with diagram 6 (page 20): –
Which scenario does the author mean us to use? Is Mars moving across the sky faster or slower than Earth? They can’t both be right. This poses serious problems for his argument.
However, the author seems to have settled diagram 20 in his mind, because he goes on to explain why he thinks that scenario, with Mars moving further across the sky in a night than Earth, is correct, because Mars moves east across the sky in its orbit anyway, according to Mr Hickson. Here he displays his ignorance to the world.
The casual stargazer will notice planets like Mars generally drift east in the sky from night to night, but the observant stargazer will eventually notice the planets change this direction and drift west for a while: this is known as
retrograde motion. This has fascinated astronomers for millennia: all the astronomers from the ancient Babylonians to the present day know about retrograde motion and Ptolemy’s
epicycles were introduced to account for it. In the course of an orbit, Jupiter and Saturn will show retrograde motion several times and Mars only once, but this happens when Earth is closest to the planet in question.
Gill made his measurements when Mars was at opposition, which is when Earth is closest to Mars, which is also when Mars is seen in retrograde motion. Had the author bothered to check Gill’s detailed reports to the Royal Astronomical Society he would have discovered this and saved himself a great deal of embarrassment.
Gill includes a chart of Mars’s position in the sky during his observations. I say observations because Gill spent months at Mars Bay, Ascension, observing and recording the planet’s position. The observations detailed start on July 31 and the last is in early October: –
There are more than two dozen stars in that chart, used over the two months plus to measure the position of Mars in the sky,
not the angle of Mars from a baseline on the ground. So many are used because, as the author says, Mars is in motion during the period of observations, but while he thinks this makes the diurnal parallax method invalid, the astronomer is actually gathering data on Mars’s orbital movement so as to separate the diurnal parallax from the orbital movement. This is standard practice in parallax measurements.
From the chart, you can clearly see Mars entering retrograde motion in early August. The scale of Right Ascension along the top and Declination on the right edge clearly show the chart top is North and right is West. When Gill made his last observation in October, Mars was about to end its retrograde motion and return to the more usual eastward drift until its next opposition, about two years later.
Gill’s report shows that the author is wrong to maintain no angles can be measured, wrong to complain the motion of Mars makes parallax measurements impossible and utterly wrong in his understanding of planetary motion. Far from “
exploded” (page 44), the “
theory of parallactic angles” is vindicated.
And the author’s cheap (and unsubstantiated) crack about there being only 7½ hours between the evening and morning observations also overlooks practical reality: there are less than 10 hours of full darkness for observations at Ascension in August, September and October, so if Mars rises later than the sun sets, there will be less than 10 hours available for two full sets of observations before the next day.
https://academic.oup.com/mnras/article/39/2/98/1018889