Cetus
The sea monster lurking in autumn skies
Best on September–November evenings · celestial equator (visible from both hemispheres).
Cetus is one of the largest constellations in the sky, sprawling across the celestial equator in the autumn sky. It represents the sea monster of Greek myth — the beast sent to menace Andromeda — and spreads wide enough that its head and tail sit in noticeably different parts of the sky. It's best known for harboring Mira, one of the most dramatic variable stars visible to the naked eye.
How to find it
On autumn evenings Cetus is well placed for observers in both hemispheres, lying just south of the ecliptic. A good way in is to find the Great Square of Pegasus high in the east, then drop south and slightly west to pick up the bright isolated star Diphda — that's the tail of the sea monster, and your foothold on the whole constellation. From Diphda, Menkar marks the head of the beast about 25 degrees to the northwest, completing the creature's long outstretched form.
Brightest stars
Diphda (β Cet) leads the constellation at a steady magnitude 2.04, making it an easy naked-eye anchor despite being the beta star. Menkar (α Cet) follows at 2.53 and marks the monster's open jaw.
Worth seeing
Mira (ο Cet) is the standout: a famous long-period variable star that swings from naked-eye visibility around magnitude 3 all the way to near-invisibility and back over roughly 11 months, making it one of the most rewarding stars to track across a season.
Frequently asked
When is Cetus visible?
Autumn evenings, September through November, are the prime time. Sitting on the celestial equator, Cetus is visible from both the northern and southern hemispheres, passing high in the south for northern observers and high in the north for southern ones.
What are the brightest stars in Cetus?
Diphda (magnitude 2.04) is the brightest, marking the tail, with Menkar (2.53) at the head. Mira, Dheneb, Kaffaljidhma, and Deneb Kaitos Schemali fill out the body at magnitudes between 3 and 3.6.
Which hemisphere is Cetus best seen from?
Both, equally well. Cetus straddles the celestial equator, so it rises and sets for observers across the globe and climbs to a reasonable height from tropical and mid-latitudes in each hemisphere during autumn evenings.
Nearby constellations
Pisces · Aries · Eridanus · Aquarius · Taurus · Andromeda · Pegasus · Perseus