Sidereal Time Calculator
ANA›Life Services Authority›National Calculator Authority›Sidereal Time Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Sidereal Time Calculator
Calculate Greenwich Mean Sidereal Time (GMST) and Local Sidereal Time (LST) for any date, time, and longitude.
Date (UTC)
Time (UTC, HH:MM:SS)
Observer Longitude (°, East positive)
Calculate Results will appear here.
function sidCalc() { var dateStr = document.getElementById('sid-date').value; var timeStr = document.getElementById('sid-time').value; var lon = parseFloat(document.getElementById('sid-lon').value); var res = document.getElementById('sid-result');
// --- Validation --- if (!dateStr) { res.innerHTML = 'Please enter a valid date.'; return; } if (!timeStr) { res.innerHTML = 'Please enter a valid time.'; return; } if (isNaN(lon) || lon 180) { res.innerHTML = 'Longitude must be between -180° and +180°.'; return; }
// --- Parse date/time into UTC components --- var parts = timeStr.split(':'); var hh = parseInt(parts[0], 10); var mm = parseInt(parts[1], 10); var ss = parts[2] ? parseInt(parts[2], 10) : 0;
var dparts = dateStr.split('-'); var year = parseInt(dparts[0], 10); var month = parseInt(dparts[1], 10); var day = parseInt(dparts[2], 10);
// --- Julian Date (JD) --- // Algorithm from Meeus, "Astronomical Algorithms", Ch.7 if (month ' + 'Julian Date (JD)' + JD_disp + '' + 'Julian Centuries (T)' + T.toFixed(8) + '' + 'GMST' + toHMS(GMST_h) + ' (' + GMST_h.toFixed(6) + ' h)' + 'LST (lon = ' + lon.toFixed(4) + '°)' + toHMS(LST_h) + ' (' + LST_h.toFixed(6) + ' h)' + ''; }
#### Formulas Used
Julian Date: JD = INT(365.25(Y+4716)) + INT(30.6001(M+1)) + D + B − 1524.5 + UT/24 where B = 2 − A + INT(A/4), A = INT(Y/100) (Gregorian calendar correction)
Julian Centuries from J2000.0: T = (JD − 2451545.0) / 36525
Greenwich Mean Sidereal Time (GMST) at 0h UT: GMST₀ = 24110.54841 + 8640184.812866·T₀ + 0.093104·T₀² − 6.2×10⁻⁶·T₀³ (seconds) where T₀ is T evaluated at 0h UT of the given day.
GMST at observation time: GMST = GMST₀ + UT × 1.00273790935 (sidereal seconds per solar second)
Local Sidereal Time (LST): LST = GMST + λ / 15 (hours) where λ is the observer's longitude in degrees (East positive).
#### Assumptions & References
- All date/time inputs are in UTC.
- GMST formula is the IAU standard from Meeus, J. — Astronomical Algorithms, 2nd ed. (1998), Ch. 12.
- This calculator computes Mean Sidereal Time; it does not apply the equation of the equinoxes needed for Apparent Sidereal Time (nutation correction ≈ ±1 s).
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