How to see a Stellar Eclipse of Algol, the Demon Star

November 11th, 2015

The Moon turns new today which means it’s a great time to go star gazing. Why? Because the Moon’s light won’t brighten the sky and wash out all but the brightest stars from the sky. One star in particular – Algol – is certainly worthy of our attention.

Algol has been known since ancient times as the “demon star.” Its light represents the eye of the Medusa, slain by the hero Perseus on his quest to save the princess Andromeda from the sea monster Cetus.

The ancients called it the demon star because, strangely and inexplicably, the star would appear to regularly brighten and fade over a period of nearly three days. This mystery remained for centuries until the late 19th century when astronomers discovered the real reason: Algol is a double star.

Unseen by human eyes, a second, dimmer star orbits the main star in the system. Every time the dimmer star passes in front of the brighter main star, the main star is eclipsed and, consequently, observers on Earth see Algol slowly fade.

Once the eclipse ends, the dimmer star moves away and Algol is seen to brighten again. This cycle repeats itself every two days, twenty hours and forty-nine minutes with each eclipse lasting for about ten hours.

You can see this for yourself. First you’ll need to find out when the next eclipse is due – you can do that by visiting

Scroll down, click on the Algol button and then click the “initialize to today” button a little below it. Your local dates and times of minima – when Algol is at its faintest – will appear in the box on the right.

Go outside the night before and take a look at Algol. How bright is it compared to the nearby stars? Then come back at minima and look again…



All Astronomical Events for November 11th, 2015


05:34 UT – The almost new Moon passes Mercury. (Mercury: 99% illuminated, magnitude -1.1, diameter 4.7”. Libra, not visible.)

17:47 UT – New Moon. (Libra, not visible.)

The variable star Algol culminates at midnight tonight. (Average magnitude: 2.1. Naked eye, Perseus, all night. See details above.)



Events in bold involve objects and/or events that are visible with the naked eye.

All times are in Universal Time (UTC). To convert the time to your timezone, click here.

Astronomical events taken from 2015: An Astronomical Year, available in Kindle and paperback formats from Amazon in the US, Canada and the United Kingdom. Night sky simulations created using Mobile Observatory for Android devices.

2015 An Astronomical Year (Kindle Edition) 2016 An Astronomical Year Paperback Cover 2016 The Night Sky Sights The Astronomical Almanac 2015-2019 Kindle Cover - Deep Sky Observer's Guide

2015: An Astronomical Year

2016: An Astronomical Year 2016: The Night Sky Sights The Astronomical Almanac (2015-2019) The Deep Sky Observer’s Guide

(Kindle & Paperback)

(Kindle & Paperback) (Kindle & Paperback) (Kindle & Paperback)


Amazon – US

Amazon – US Amazon – US Amazon – US Amazon – US
Amazon – UK Amazon – UK  Amazon – UK Amazon – UK

Amazon – UK

Details of all available books across the world can be found here or by visiting the author’s page on Amazon. If you have any comments or questions, please feel free to contact me at astronomywriter “at”


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