NASA’s October skywatching guide points to Algol in the northeastern sky

NASA’s new October skywatching notes direct observers toward Algol, a star in Perseus that can be found in the northeastern sky during autumn beginning after 9 p.m. The guidance, published on 28 September, uses nearby Cassiopeia and Andromeda as signposts. For readers looking beyond the brightest planets, it offers a way to identify a star whose light changes noticeably over time.

The time and direction in NASA’s guide are general. It does not give a viewing location or date-specific rise times, so the view from an individual home will depend on local conditions. A separate October guide from the University of Arizona’s Flandrau Science Center & Planetarium broadly supports the route across the sky: it places Cassiopeia in the northeast, Perseus below it and Andromeda in the east-northeast at the beginning of the night.

Start with Cassiopeia’s W

Cassiopeia is the easiest landmark in the route described by the guides. Flandrau calls its W shape easy to spot in October’s northeastern sky. From there, observers can look below it for Perseus and toward the east-northeast for Andromeda. NASA advises using Cassiopeia and Andromeda to locate Perseus, then using Almach, a star in Andromeda, as a guide to Algol.

High Point Scientific’s October astronomy guide also describes Cassiopeia as a distinctive W and says it is visible throughout the month for most observers. According to that guide, it rises in the early evening and reaches its highest point in the northern sky around midnight. That gives observers another opportunity later in the evening if the constellation is awkwardly placed when they first go outside.

Why Algol changes brightness

NASA calls Algol the ‘Demon Star’ or ‘The Ghoul’ and notes its association in Greek mythology with Medusa’s blinking eye. The apparent blink has a physical explanation: Algol is a triple-star system, and two of its stars orbit one another as an eclipsing pair. As their positions change, the light seen from the system dims and brightens.

According to NASA, Algol changes from magnitude +2.1 to +3.4 about every three days. Astronomers use lower magnitude numbers for brighter objects, so the move to +3.4 means Algol has dimmed. NASA’s accompanying light curve, made with data from its Transiting Exoplanet Survey Satellite, shows that recurring change. The satellite data are held at NASA’s Mikulski Archive for Space Telescopes.

The change gives skywatchers something to compare across evenings, but NASA does not specify a minimum-brightness time for a particular October night in the article. A single glance may therefore catch Algol at any point in its cycle. NASA points readers to resources for calculating when its dimming reaches a minimum if they want to plan an observation around that moment.

For a visual comparison, NASA suggests looking at Algol alongside Almach in Andromeda through binoculars or a small telescope. The agency says the stars have similar brightness, making Almach a useful reference while Algol changes. Flandrau separately identifies Algol as a variable star and lists its usual apparent magnitude as 2.09, close to the brighter value in NASA’s account.

What the guides can establish

The three guides agree on the broad October picture: Cassiopeia is a prominent northeastern landmark, Perseus lies nearby, and Andromeda helps complete the route to Algol. High Point Scientific also recommends Cassiopeia as a marker for locating other objects near it, including the Andromeda Galaxy. Those wider sky references provide orientation, while NASA’s newly published notes supply the Algol-specific observing instructions and explanation of its changing light.

None of the guides promises a clear view from every location on a given evening. NASA’s after-9-p.m. advice has no stated latitude, and the sources do not establish the weather, horizon or sky darkness a reader will encounter. Observers can use the W of Cassiopeia and NASA’s Almach comparison as starting points, then check a location-specific sky chart before choosing when to watch for Algol’s next dip.