Track Mercury’s greatest elongations or any moment’s separation. Enter coordinates manually, or estimate from elements. Clear outputs help you spot Mercury at twilight easily.
| Date & time | UTC offset | Elongation | Direction | Note |
|---|---|---|---|---|
| 2026-01-05 18:00 | +0 | 25.2° | East | Good twilight target |
| 2026-01-12 06:00 | +0 | 15.8° | West | Low before sunrise |
| 2026-01-20 18:00 | +0 | 8.9° | East | Hard near the Sun |
| 2026-02-02 06:00 | +0 | 22.7° | West | Better morning view |
| 2026-02-10 18:00 | +0 | 12.4° | East | Possible with clear horizon |
Elongation is the angular separation between Mercury and the Sun as seen from Earth. When you have coordinates for both objects, compute the separation using spherical trigonometry.
For RA/Dec, use α for right ascension and δ for declination in the same equation. Direction (East/West) is estimated from the longitude difference sign.
Mercury elongation is the angle between Mercury and the Sun in Earth’s sky. A larger angle usually means Mercury is farther from the Sun’s glare and easier to locate. Small elongations place Mercury deep in bright twilight, while larger elongations create a wider viewing window.
Mercury never strays far from the Sun because its orbit is close to the Sun at about 0.387 AU. Greatest elongations commonly fall around 18° to 28°. The exact maximum varies with Mercury’s changing distance from the Sun and the tilt of its orbit relative to Earth.
If Mercury is east of the Sun, it sets after the Sun and appears in the evening sky. If Mercury is west of the Sun, it rises before sunrise and is a morning object. This calculator reports east or west so you can choose a sunset or sunrise session confidently.
Mercury orbits the Sun in about 88 days, while Earth takes about 365 days. From Earth, Mercury’s appearance repeats on a synodic cycle of roughly 116 days. That is why several good elongation opportunities happen each year, alternating between morning and evening.
A 22° elongation can still be difficult if Mercury sits low on your horizon. The ecliptic’s tilt at your latitude and season controls how steeply Mercury climbs after sunset or before sunrise. A clear, unobstructed horizon often improves success more than minor angle differences.
With RA/Dec, elongation is found from equatorial coordinates, a system tied to Earth’s rotation axis. With ecliptic longitude and latitude, elongation is computed in the plane of Earth’s orbit. Both methods use the same spherical separation idea, just with different coordinate inputs.
The date method uses simplified orbital elements to estimate Mercury and Sun positions. It is excellent for education, planning, and quick checks, but it is not a replacement for high-precision ephemerides. For photography or strict timing, cross-check with a trusted observatory or planetarium application.
Visibility improves as elongation grows, but sky brightness and altitude still decide the outcome. Around 10° to 18° you may need a very clear horizon. Around 18° to 24° Mercury is often easier in twilight. Beyond about 24° is usually a strong opportunity.
1) What units does elongation use?
Elongation is an angular measurement. This tool can display it in degrees, radians, arcminutes, or arcseconds. The underlying calculation is an angle between Mercury and the Sun as seen from Earth.
2) Is a larger elongation always better?
Usually, yes, because Mercury is farther from the Sun’s glare. However, Mercury can still be low on the horizon. A steep ecliptic angle and a clear horizon can matter as much as the elongation value.
3) Why does the calculator say east or west?
East means Mercury is to the east of the Sun and tends to be visible after sunset. West means it is to the west of the Sun and is usually visible before sunrise, depending on local horizon and season.
4) What does the “greatest elongation search” do?
It scans a time window around your chosen date and finds the maximum elongation using the same approximate model. It is a planning aid for locating the best moment in that window, not a precision ephemeris.
5) Can I use RA/Dec from an astronomy app?
Yes. Copy Mercury and Sun RA/Dec values from the same source and time. Keep both in the same coordinate system and epoch as provided. The calculator converts RA in hours to degrees when needed.
6) Why do my results differ from a planetarium program?
This page uses simplified orbital elements and does not model every correction. High-end tools may include light-time, aberration, perturbations, and topocentric effects. Small differences are normal, especially near small elongations.
7) Is it safe to search near the Sun?
Be careful. Never stare at the Sun, and never aim binoculars or a telescope near it without proper solar safety. Use safe viewing practices, and wait until the Sun is fully below the horizon for evening sessions.
Never look at the Sun directly or through optics. Use safe solar filters and follow local guidance. Mercury is always close to the Sun, so choose a clear horizon and observe responsibly.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.