The Andromeda Galaxy (Messier 31 or M31) is the closest large spiral galaxy to the Milky Way, residing approximately 2.5 million light-years away. It contains an estimated one trillion stars and spans over 150,000 light-years in diameter. M31 is the most distant object in the cosmos visible to the unaided human eye under optimal conditions. Locating this celestial neighbor requires knowing the right time to look and using specific star patterns as guides.
When is the Best Time to View
The optimal time to view the Andromeda Galaxy from the Northern Hemisphere is during the autumn and early winter months (September through December). During this period, the galaxy is highest in the sky, allowing viewers to look through the least amount of Earth’s distorting atmosphere. Viewing an object near the zenith, or directly overhead, provides the clearest possible image.
The best time of night to search is typically between 10:00 PM and midnight, when the galaxy reaches its highest point. M31 rises earlier in the evening as the season progresses from late summer into autumn. To ensure the faint light of the galaxy is not overpowered, observe during the new moon phase or the week following the last quarter moon. This minimizes moonlight that can wash out the sky.
Identifying the Guide Constellations
Locating M31 relies on star-hopping, a technique that uses brighter, easily identifiable star patterns as celestial landmarks. The two primary guides for finding Andromeda are the constellations Cassiopeia and Pegasus. Cassiopeia is a distinctive pattern that looks like a large ‘W’ or ‘M’ shape, depending on its orientation.
The other starting point is the Great Square of Pegasus, a prominent asterism forming the body of the winged horse constellation. The constellation Andromeda is situated between these two constellations. Andromeda itself begins at Alpheratz, one of the stars of the Great Square, and extends away from Pegasus in a curving chain of dimmer stars.
Step-by-Step Location Guide
Finding M31 often involves a two-part star-hop starting from the Great Square of Pegasus. First, locate the bright star Alpheratz, which forms the northeast corner of the Great Square. Follow the line of stars that curves away from the square, representing the constellation Andromeda.
The second star along this line is Mirach (Beta Andromedae), a moderately bright star that serves as an important pointer. Draw an imaginary line from Mirach to the fainter star Mu (µ) Andromedae. The Andromeda Galaxy is located by continuing this line past Mu Andromedae for roughly the same distance that separates Mirach and Mu.
Alternatively, the ‘W’ shape of Cassiopeia can be used as a pointer. Focus on the deeper ‘V’ of the ‘W’ shape; the two stars forming the bottom of that ‘V’ point directly toward Mirach. Following this line brings you close to the galaxy’s location, allowing you to use the Mirach-to-Mu star-hop for the final identification.
Viewing Conditions and Tools
Under a truly dark sky, the Andromeda Galaxy appears to the naked eye as a small, faint, elongated smudge of light. This subtle appearance is due to its distance of 2.5 million light-years, making it look like a hazy oval. Using averted vision, where you look slightly to the side of the galaxy, helps the eye’s more sensitive peripheral rods detect the extended glow.
While a telescope can offer higher magnification, the Andromeda Galaxy is immense in angular size—about six times the apparent diameter of a full moon. Therefore, a telescope often only shows the bright central core. For the best overall view that captures the full extent of the galaxy, a pair of binoculars is recommended. Binoculars offer a wider field of view and gather significantly more light than the naked eye, transforming the faint smudge into a noticeable fuzzy patch. Locating a viewing spot far from city lights is essential, as light pollution is the largest obstacle to seeing the galaxy’s delicate details.