John D. Sabia
Original text from Sky and Telescope web page follows.
The star Algol (b Persei) was the first eclipsing variable star ever discovered, and it's still the most famous one. You can check on it whenever you step outdoors on nights when Perseus is in view.
Algol fades and rebrightens like clockwork every 2.87 days. Its changes are very plain to the naked eye. In the middle of an eclipse it shines dimly at magnitude 3.4 instead of its usual 2.1.
Algol stays nearly that faint for two hours centered on the time of mideclipse, and it takes several additional hours to fade and to rebrighten.
Good comparison stars are Gamma (g) Andromedae to Algol's west, magnitude 2.1, and Epsilon (e) Persei to its east, magnitude 2.9. You can compare Algol's brightness with them at a glance; click on the star chart to see a larger version with the magnitudes of several comparison stars clearly labeled. (For detailed tips on estimating a variable's brightness, see "The Lure of Variable Stars." For information and finder charts for Algol and 11 other inconstant stars, see "The Top 12 Naked-Eye Variable Stars.")
Below is a calculator you can use to predict when Algol or other EB will be at mideclipse. Press "Initialize to today" to view the dates and times of Algol's minima for the next three weeks. Or you can enter any date, between 2007 and 2015 inclusive, to see the dates and times of eight consecutive minima. The times given should be accurate to within a few minutes.
The second row of stars will calculate time of Maxima light of selected Cepheid variable.
|EB - Eclipsing Binaries|