The positions of planets relative to the Sun and the Earth are called configurations. For planets inside the Earth’s orbit (Mercury, Venus): inferior conjunction (planet between Earth and Sun), superior conjunction (behind the Sun), and the greatest elongations – positions where the planet’s angle from the Sun is greatest (Mercury up to ≈ 28°, Venus up to ≈ 47°). So these planets are seen only in the morning or evening twilight near the Sun and are never on the opposite side of the sky at midnight. For outer planets (Mars and beyond): conjunction (behind the Sun – not visible), opposition (Earth between Sun and planet – the planet is visible all night and closest) and the eastern and western quadratures (90° from the Sun). Around opposition outer planets briefly move backward against the stars (retrograde motion) because the Earth overtakes them. When observing Mercury and Venus, never point binoculars or a telescope towards the Sun while the Sun is above the horizon.
Worked examples
At greatest elongation Venus is 47° from the Sun; since the sky turns 15° per hour it rises or sets at most ≈ 47 : 15 ≈ 3 hours before or after the Sun (the exact time depends on how the ecliptic is tilted to the horizon).
Jupiter at opposition rises as the Sun sets, is on the meridian at midnight and sets at dawn – visible all night. Venus at inferior conjunction is lost in the Sun’s glare and is not visible.
Class activity
Using a planetarium app, find which planets are visible in the evening and which before dawn this month and tabulate them. Do real observing only with an adult and after the Sun has fully set.
Practice
1
What is opposition and for which planets does it exist?
The position with the Earth between the Sun and the planet; only for outer planets.
2
If Mercury is at an elongation of 30° (rounded), how many hours before or after the Sun does it rise or set (15° per hour)?
2
3
Why is Venus never seen at midnight?
Its orbit lies inside the Earth’s and it never gets more than 47° from the Sun.
4
Why does Mars appear to move backward around opposition?
The faster Earth on its inner orbit overtakes Mars and the line of sight shifts against the stars.