Almanac · Planets
Mercury at greatest eastern elongation, 28 May 2027
Mercury stands 22.9° east of the Sun on Fri 28 May 2027 at 10:00 UTC, the widest it gets in this evening showing: look for it low in the west after sunset. How high that puts it depends on your latitude and the season, and the table below works it out.
- Greatest elongation In the evening sky, low in the west after sunset
- Brightness +0.5 Magnitude: lower is brighter; the brightest stars are near 0
- From the Sun 22.9° east 37% 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 | 22:20 | 6° down | Low: 4° up in the NW at 22:20; a clear, flat horizon needed |
| 45°N | 20:20 | 7° down | 10° up in the WNW at 20:20 |
| 30°N | 19:30 | 8° down | 14° up in the WNW at 19:30 |
| 15°N | 18:50 | 6° down | 17° up in the WNW at 18:50 |
| 0° | 18:30 | 8° down | 15° up in the WNW at 18:30 |
| 15°S | 18:00 | 6° down | 14° up in the WNW at 18:00 |
| 30°S | 17:40 | 7° down | Low: 10° up in the NW at 17:40; a clear, flat horizon needed |
| 45°S | 17:10 | 7° down | Low: 4° up in the NW at 17:10; a clear, flat horizon needed |
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