Hydra
looking toward
SSE153°
ALT 21° · FOV 93°
🌟ByulGil
observatory
Seoul
37°33′N · 126°58′E
2026.10.22 18:53
LIVE · 24 OF 40 ABOVE
Tonight's Sky
40 CONSTELLATIONS
constellation

Hydra

바다뱀자리
The longest constellation in the sky · The Water Snake
Spring · stretching long across the southern sky
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Science
The science

Hydra is the largest and longest of all 88 constellations, winding across more than 100 degrees of sky (about a quarter of the whole). So long is it that hours pass between the rising of its western head and the appearance of its eastern tail. For its size it has few bright stars; its only 2nd-magnitude star, the chief star Alphard (meaning "the solitary one"), glows alone and red in the southern spring sky. Near its head lie the beautiful spiral galaxy M83 and the open cluster M48.

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Mythology
The myth

Hydra is the monstrous water serpent that appears in Heracles' second labor. This beast, dwelling in the swamp of Lerna, had nine heads, and it was immortal: cut off one and two new heads sprang up in its place. With the help of his nephew Iolaus, Heracles seared each neck with fire as he struck off its head, so that no new head could grow, and he sealed the last, immortal head beneath a great boulder. The crab Hera sent to hinder the fight became Cancer, and the fallen Hydra became this constellation, the two hung side by side in the spring sky.

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Stars
Main stars (equatorial)
EpsilonRA 8.78h · Dec 6.4° · m3.4
ZetaRA 8.89h · Dec 6.0° · m3.1
AlphardRA 9.46h · Dec -8.7° · m2.0
NuRA 13.32h · Dec -16.2° · m3.1
GammaRA 13.83h · Dec -23.2° · m3.0
PiRA 14.72h · Dec -26.7° · m3.3
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What is Byulgil?

Byulgil is a stargazing web app that computes the real night sky for your location, anywhere in the world, in real time. Byulgil means "Star Loads", in Korean. Use your current location (GPS) or pick a major city — Seoul, Tokyo, New York, Sydney — to see the sky exactly as it appears there, right now. Star positions come from real J2000.0 equatorial coordinates, converted to horizontal coordinates for your time and place. On mobile, AR mode lets you point your phone at the sky to overlay the constellations in that direction.