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Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star" most of the time it shines at magnitude +2.1 but every two days, 20 hours and 49 minutes the star suddenly dips in brightness to magnitude +3.4, remaining dim for about 10 hours before returning to its original state.

Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary but this results in a very small dip in brightness that can only be detected by those with access to photo-electrical equipment.

Algol System (credit - Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)

Algol is located in Perseus among the stars of the northern Milky Way. It's positioned west of magnitude +0.1 star Capella (α Aur) and southeast of the well known "W" of Cassiopeia. The finder chart below shows the position of Algol along with magnitude data of some surrounding stars for comparative purposes.

Finder Chart for Algol (credit:- freestarcharts)

Finder Chart for Algol (credit:- freestarcharts) - pdf format

For those new to variable star observing, Algol is an excellent first choice target. It dims quite dramatically and the regular brightness changes are obvious to the naked eye. The table below contains date and time information of Algol's minima for July 2016 for various World time zones that cover most regions of Europe, America, China and Australia. For time zones not listed, local eclipse times can be calculated by adjusting the table to fit to the observers local time.

Algol Minima for July 2016

DateTime (UT-GMT)Time (BST-UK)Time (CEST-Europe)Time (EDT-N America)Time (PDT-N America)Time (CST-China)Time (AEST-Australia)
July 0300:1401:1402:1420:14 (Jul 02)17:14(Jul 02)08:1410:14
July 0521:0322:0323:0317:0314:0305:03 (Jul 06)07:03 (Jul 06)
July 0817:5118:5119:5113:5110:5101:51 (Jul 09)03:51 (Jul 09)
July 1114:4015:4016:4010:4007:4022:4000:40 (Jul 12)
July 1411:2812:2813:2807:2804:2819:2821:28
July 1708:1709:1710:1704:1701:1716:1718:17
July 2005:0606:0607:0601:0622:06 (Jul 19)13:0615:06
July 2301:5502:5503:5521:55 (Jul 22)18:55 (Jul 22)09:5511:55
July 2522:4323:4300:43 (Jul 26)18:4315:4306:43(Jul 26)08:43 (Jul 26)
July 2819:3320:3321:3315:3312:3303:33 (Jul 29)05:33 (Jul 29)
July 3116:2117:2118:2112:2109:2100:21 (Aug 1)02:21 (Aug 1)