Autumn in the northern hemisphere is a good time of the year to see Perseus in the evening sky. But rather than simply searching out the constellation, how about watching the star Algol change in brightness?
This Saturday, from the UK and Europe the star will be at its dimmest during the late evening. According to myth, Algol represents the eye of Medusa, the gorgon slain by Perseus, whose head the hero holds in his left hand.
Algol is in fact two stars in orbit around one another, too close to be distinguished separately. Every 2.87 days, the dimmer star passes in front of the brighter star and blocks out some of its light. When it does, Algol’s brightness plunges by a factor of 3.3.
The easiest way to notice this is to compare it with nearby stars. Almach in Andromeda shines at magnitude 2.1, Algol’s usual brightness. During eclipse, however, Algol drops to magnitude 3.4, similar to Epsilon Persei (magnitude 2.9).
At 22.30 BST on 18 September, Algol will be in the middle of its eclipse, and comparable to Epsilon Persei. On Friday and Sunday, it will be the same brightness as Almach. This website will calculate local eclipse times for Algol across the world.