The tortuous discovery of the Gegenschein, false zodiacal light & zodiacal band – Brorsen vs Jones

2024 February 8

Marinus A. van der Sluijs

It is routinely thought that Brorsen discovered the Gegenschein and zodiacal band in 1854. Here, it is argued that priority for the Gegenschein should go to Jones. Both also observed the false zodiacal light, which von Humboldt had seen long before. As that was not recognised as a separate phenomenon until c. 1950, Brorsen imagined it to be a part of the Gegenschein and Jones of the band, thus causing much confusion in concepts and nomenclature.

The Gegenschein

The Gegenschein (German for ‘countershine’) is a diffuse glow in the night sky caused by the backscattering of sunlight off interplanetary dust outside Earth’s orbit. Situated exactly at the antisolar point (ASP) in the ecliptic, it is effectively a ghostly mirror image of the Sun or ‘antisun’ that changes its position with the Sun’s invisible passage below the horizon. Best seen around midnight, when it appears highest in the sky, in months when it is not in front of the Milky Way,¹ it varies in shape from ‘small and somewhat elongated’ to ‘very large and round’.²

The Danish astronomer Theodor Johan Christian Ambders Brorsen (1819–1895) is best known for his discovery of no less than five comets in the period 1846–1851, but in a recent article Donald Olson also identified him as the first person to have described the Gegenschein – in 1854,³ with the first sighting dated to April 17 of that year.⁴ Earlier, Fechtig et al. (2001),⁵ Roosen (1971) and others had reached the same conclusion.⁶ Olson laid to rest the occasional claim that the French savant Esprit Pezenas (1692–1776) had made an earlier observation of the phenomenon on 1730 February 15, as that doubtless concerned a type of auroral arc. However, George Jones (1800–1870) arguably observed the Gegenschein a couple of months before Brorsen did so.

Jones was an American Navy chaplain, whose pioneering studies on the zodiacal light are well known. His monumental report of observations made in 1853–1855 on board the steam frigate Mississippi, published in 1856, made an invaluable contribution to the field, despite its erroneous contention that the zodiacal dust cloud is centred on Earth. In later articles integrating additional work in the Ecuadorian Andes, Jones reflected that he had looked in vain on that occasion for Brorsen’s Gegenschein.⁷ It does not seem to have occurred to him that he may have spotted it unknowingly during his earlier journey, as borne out by two passages in his monograph. For the morning of 1854 January 30, while travelling at 26° 10ʹ N, Jones had written that he was at a loss to account for a feature in the night sky:

‘There is a broad streak of sky from Regulus up to the Milky Way …, which puzzles me. I cannot make out whether its peculiar appearance is owing to the Zodiacal Light, or to a want of stars and a steady paleness or dimness there. From Præsepe up, however, it seems to amount almost or quite to a positive light, like the Diffuse Zodiacal Light. At 3^h, 4^h, and 5^h, however, I could not see it higher than nearly to Regulus. But these palenesses are all so indefinite, that it is often difficult to get their boundaries …’⁸

The observation was made between 2 and 5 a.m., while the sky was ‘very clear’. About two weeks later, on the evening of February 15, Jones was at 35° 19ʹ N and commented on what must be the same mysterious glow:

‘There is a regular paleness of the sky from Regulus, up by Præsepe, &c., to the Milky Way, and about 8° wide; its centre nearly or quite on the ecliptic. It amounts almost, if not quite, to a positive light, and seems like a dim branch of the Milky Way, that has strayed off from the general course.’⁹

In 1932, Cuno Hoffmeister noted that this passage describes the Gegenschein ‘quite accurately’: ‘The centre of the Gegenschein should be located slightly east of Regulus.’ Hoffmeister added the zodiacal band (see below) as a possible referent, but with a caveat: ‘Because the observation took place in the evening towards 7 o’clock, the luminous band could not be seen further to the east.’¹⁰ Nothing in Jones’ wording warrants a role for the zodiacal band, but the timing of around 7 p.m. can be confidently inferred from the fact that the Moon rose soon after and Jones turned his attention away from the zodiacal light in the west to the eastern horizon in order to ‘catch the first appearance of the moon’s Zodiacal Light’. While the latter notion has long been discredited, the Gegenschein would then be in that part of the sky, some distance above the horizon, and could only be seen in the Moon’s absence. An appearance around 7 p.m. is early indeed for the Gegenschein, but the location of the ‘paleness’ relative to the stars was identical to that in the early morning hours of January 30 and Jones did state that it ‘was a very fine evening’, with the ‘sky remarkably clear and good’. The two reports of the ‘paleness’ contain enough detail to be confident that Hoffmeister’s main assessment is spot on. Jones’ size estimate of the spot, too, is realistic for the Gegenschein. This would make these reports the earliest known scientific descriptions of the Gegenschein, even if Jones was unaware that he had recorded a novel phenomenon.

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