Olbers, his comet and his Paradox

Now comes the fourth of the five comets of 2024 that we listed in January.

Arrows through the moving bodies, including the Sun, show their movement (against the starry background) from 2 days earlier to 2 days later.

The comet is now conveniently placed; at the location and time of this scene it is about 20° above the horizon. But its magnitude of about 7.6 is dimmer than the naked-eye threshold of about 5 or 6. In binoculars, it may show a tail sweeping back as the comet speeds around the inner curve of its orbit.

This space view, from 15° north of the ecliptic plane, shows the path of the comet in 2024, with stalks to the plane at 1-month intervals. Red lines are at the dates when the comet is closest to the Sun and the Earth. Grid lines on the ecliptic plane are at intervals of 1 AU (astronomical unit, Sun-Earth distance, about 150 million km or 93 million miles). Earth is exaggerated 500 times in size, the Sun only 5 times.

As its designation shows, 13P Olbers was the 13th comet recognized to be periodic, that is, returning in less than 200 years..

But it is an example of the Halley-type subgroup, with a period longer than 20 years: 69, compared with 76 for 1P Halley itself. In other ways the orbit is unlike that of Halley’s Comet, which crosses inward of Earth’s orbit. Comet Olbers comes to a perihelion about 1.2 AU from the Sun, and far north of our orbit.

It ascended north through the ecliptic plane on Apr. 15 – not far from the orbit of Mars, on which its tail may cause meteors showers.

The orbital elements have changed slightly since January, causing the comet to arrive at perihelion on June 30 rather than July 4. The brightness may still be increasing slightly.

Here is the comet’s path through the constellations in the coming weeks. It will be northernmost, at declination 44°20′, on June 30.

Heinrich Wilhelm Mathias Olbers (1758-1840) discovered this comet on 1815 March 6, nearly two months before its perihelion. It has been observed at its returns in 1887 and 1956. This time, it was recovered by Alan Hale on 2023 Aug. 24.

Why are there holes between the stars?

Olbers was a medical doctor by day and an astronomical observer and thinker by night. He also discovered non-periodic comets C/1780 Montaigne-Olbers and C/1796 Olbers, and asteroids 2 Pallas (1802) and 4 Vesta (1807).

Olbers’s paradox, or the dark night paradox, was described by him in 1823, though it had been in essence suggested by Thomas Digges, Johannes Kepler, Edmond Halley, and Jean-Philippe de Cheseaux.

The night sky should be bright all over, if stars are homogeneously distributed in an infinite and static universe. A sightline in any direction would end at the surface of a star.

Distance alone does not explain it (brightness decreases by the inverse square law): at 2 times the distance, stars appear 1/4 as bright, but there would be 4 times as many stars.

The paradox has stimulated theorists (including Lord Kelvin and Edgar Allan Poe). The chief factor that resolves it is that the universe is not static but expanding. The light from stars moving away is “red-shifted” into longer wavelengths, below the range of visible light.