Moon-Mercury-Pleiades Conjunction

Honourable mention in the 2022 IAU OAE Astrophotography Contest, category Still images of celestial patterns.

This photograph shows the young lunar crescent, some of the nine brightest stars of the Pleiades (with one behind a cloud) on the right, and the planet Mercury, looking slightly red, in the middle of the image.

This picture is rather suggestive of the idea that the Pleiades might possibly consist of seven stars. However, the viewer is misled by the clouds; five of the stars form a tiny chariot, one is next to the handle, and three are at the other end of the quadrilateral. Eight stars would be clearly visible if there were no clouds.

This configuration of the young Moon next to the Pleiades is visible only in the northern hemisphere spring. Thus it was used by the ancient Babylonians to determine the second month of their year and to judge whether or not an intercalary month was necessary. At least as early as the second millennium before the common era, the Babylonians used several asterisms for each month, with another one of them reappearing every five days after invisibility during daylight. To determine the necessity of intercalation in order to synchronise the solar and the lunar year, the Babylonians used several asterisms, not only the Pleiades. For instance, they also made use of the bright stars Arcturus and Sirius, and they observed a configuration with the Moon as well as heliacal phenomena. The modern Jewish and modern Islamic traditions still make use of some of the Babylonian astronomical rules. However, given that the constellations have shifted as a result of precession, and the fact that nowadays we also have computational means to calculate our calendars, this configuration of the small crescent Moon and the Pleiades is less useful, though it remains exceptionally beautiful. Thus the ancient Babylonian and middle Babylonian tradition survives only rudimentarily. Furthermore, it is unlikely that it is depicted in the Nebra Disc from Bronze Age Europe, as has long been claimed.

This image was taken on Elba Island, Italy, in May 2022.

Credit: Giulio Colombo/IAU OAE (CC BY 4.0)