Almanac · Planets
Mercury at greatest western elongation, 21 Apr 2025
Mercury stood 27.4° west of the Sun on Mon 21 Apr 2025 at 18:50 UTC, the widest it gets in this morning showing: look for it low in the east before sunrise. How high that puts it depends on your latitude and the season, and the table below works it out. The next is Mercury at greatest western elongation, 20 Nov 2026.
- Greatest elongation In the morning sky, low in the east before sunrise
- Brightness +0.4 Magnitude: lower is brighter; the brightest stars are near 0
- From the Sun 27.4° west 46% of its disc lit, as a telescope shows it
Latitude by latitude
At each latitude, the moment within 12 hours of the event, with the Sun at least 6° below the horizon, that Mercury stands highest, and how high. Times are local solar time: your clock may differ by up to an hour or two, by your time zone.
| Latitude | Best moment | The Sun then | What you see |
|---|---|---|---|
| 60°N | — | — | It is below the horizon whenever the sky is dark |
| 45°N | 04:30 | 6° down | Lost in the horizon's haze: 1° up at best once it is dark |
| 30°N | 05:00 | 6° down | Low: 8° up in the E at 05:00; a clear, flat horizon needed |
| 15°N | 05:10 | 8° down | 12° up in the E at 05:10 |
| 0° | 05:30 | 7° down | 17° up in the E at 05:30 |
| 15°S | 05:40 | 8° down | 19° up in the E at 05:40 |
| 30°S | 05:50 | 8° down | 19° up in the E at 05:50 |
| 45°S | 06:10 | 7° down | 19° up in the ENE at 06:10 |
How we worked it out: the planets are placed by VSOP87, the planetary theory the map draws, within about an arcsecond of JPL’s ephemeris, and the Sun by Meeus’s solar theory. Greatest elongation is the instant of Mercury’s widest angle from the Sun, measured across the sky rather than along the ecliptic, found to 10 minutes. Each latitude is walked every 10 minutes over the 12 hours either side, at Greenwich’s longitude. The tests hold the dates to the U.S. Naval Observatory’s and the published calendars’, within a day, and the angles within 0.3°.
- P. Bretagnon and G. Francou, VSOP87, the planetary theory the map draws, held to JPL’s DE441 within about an arcsecond over 1900–2100
- U.S. Naval Observatory and Sea and Sky’s Astronomy Calendar of Celestial Events: the dates and angles the tests hold these to
- A. Mallama and J. Hilton (2018) and the Astronomical Almanac’s phase-angle series: the planets’ magnitudes
- J. Meeus, Astronomical Algorithms, ch. 25: the Sun, for the twilight each latitude is judged in