🔭 Astronomy & Celestial Mechanics
Sidereal vs Solar Time ($\text{GMST} \leftrightarrow \text{LST}$) Converter
Convert Universal Coordinated Time ($\text{UTC}$) to Greenwich Mean Sidereal Time ($\text{GMST}$) and Local Sidereal Time ($\text{LST}$) for celestial telescope pointing and Right Ascension ($\text{RA}$) alignments.
🕰️ Greenwich (0.0°)
🌋 Mauna Kea (-155.47°)
🇨🇱 Paranal VLT (-70.40°)
🗼 Tokyo (139.75°)
⏱️ Use Current Date/Time
Observer Date & Location
Positive (+) = East, Negative (-) = West.
Local Sidereal Time (LST)
00h 00m 00s
GMST (Hours)
0.000 h
LST (Degrees)
0.000°
Julian Date (JD)
2460000.0
💡 Celestial Right Ascension Rule:
$$\text{Objects at the local meridian have Right Ascension } \alpha = \text{LST}$$
Understanding Sidereal Time vs Solar Time
Sidereal time is a time scale based on the rate of Earth's rotation measured relative to fixed background stars rather than the Sun:
- The Sidereal Difference: Because Earth orbits the Sun, it must rotate approximately $361^\circ$ to bring the Sun back to the zenith (solar day = 24h), but only $360^\circ$ relative to fixed stars: $$1\text{ Sidereal Day} \approx 23\text{h } 56\text{m } 4.0905\text{s Solar Time}$$
- Local Sidereal Time (LST): $$\text{LST} = \text{GMST} + \frac{\text{Observer Longitude (degrees)}}{15^\circ/\text{hour}}$$
- Telescope Pointing: A star with Right Ascension ($\alpha$) crosses the observer's local celestial meridian when $\text{LST} = \alpha$.