Planetary aspects Greatest elongation
Greatest elongation
The days when Mercury and Venus are easiest to find
An inner planet reaches its greatest angle from the Sun, and its easiest visibility.
- Mercury’s greatest elongation
- About 18° to 28°
- Venus’s greatest elongation
- About 45° to 47°
- Eastern elongation
- Low in the west after sunset
01 / Overview
Meet Greatest elongation
Mercury and Venus orbit inside Earth’s, so from here they only ever swing back and forth near the Sun. The extremes of that swing are the greatest elongations: at eastern elongation the planet stands low in the west after sunset, at western elongation low in the east before sunrise.
Why Mercury is so hard to see
Mercury’s greatest elongation is only 18° to 28°, which keeps it in twilight, low down and dimmed by the thick air near the horizon. Venus reaches about 47° and can hold a dark sky for hours, which is how it became the familiar morning and evening star.
Elongation and phase
The inner planets show phases too. Near greatest elongation a telescope shows a half-lit disc; the closer the planet comes to inferior conjunction, the nearer and larger it grows while the lit part narrows, until only a thin hook is left.
02 / Key data
Core parameters
- Mercury’s greatest elongation
- About 18° to 28°
- Venus’s greatest elongation
- About 45° to 47°
- Eastern elongation
- Low in the west after sunset
- Western elongation
- Low in the east before sunrise
- Recurrence
- Several times a year for Mercury, about every 19 months for Venus
Data and calculation notes
Active periods, peak dates and rates are multi-year averages; each year differs a little, so plan against the forecast issued for the year in question.
03 / Observing tips
How to watch
- Look about 40 minutes after sunset or before sunrise, from a site with an open horizon.
- Venus reaches magnitude −4 and can be picked up with a telescope in daylight.
- Never sweep with optics while the Sun is still above the horizon.
04 / Sources