Make a star map poster of the night sky on your date

How the sky is calculated

Each star in the catalog is located by its right ascension and declination. To find where it sits in your sky, the tool converts your local time to universal time, then computes the local sidereal time: the right ascension crossing the meridian at that moment.

\[ \begin{gathered} \text{GMST} = 280.4606^{\circ} + 360.9856^{\circ} \times (\text{JD} - 2451545.0) \\ \text{LST} = \text{GMST} + \text{longitude} \\ \text{hour angle} \ H = \text{LST} - \text{right ascension} \\ \sin(\text{altitude}) = \sin(\delta) \cdot \sin(\varphi) + \cos(\delta) \cdot \cos(\varphi) \cdot \cos(H) \end{gathered} \]

Precession of the equinoxes is corrected for the chosen date, which matters for a 1950 or 2030 sky. The zenith is at the center of the disk and the horizon forms the edge, with north at the top and east on the left, just as when you tilt your head back and look up.

What the map includes

• More than 2,000 stars down to magnitude 5.2, the ones you can see with the naked eye under a clear sky.
• The figures of 88 constellations, labeled in English.
• Names of the brightest stars: Sirius, Vega, Arcturus, Rigel and more.
• Compass points and a horizon scale.

Which place and time to use

For a birth, use the hospital's city and the time on the birth certificate. For a wedding, the ceremony. The time zone offset, including daylight saving, is worked out automatically; if you know the exact UTC time you can edit it. If you don't know the time, 10 p.m. gives a lovely autumn or winter sky. The tool also shows the local sidereal time so you can check the math.