The difference between sidereal and synodic (solar) days and months, the lunar phase cycle, and the astronomical significance of equinoxes and solstices.

What this dot point is asking

Edexcel statements 4.1 and 4.9 to 4.14 want you to understand the difference between sidereal and synodic (solar) days and months, the lunar phase cycle, the astronomical significance of equinoxes and solstices, and the variation in the Sun's apparent motion through the year.

Sidereal versus synodic days

This is a core explanation question. After one sidereal day the stars are back where they started, but the Sun is not, because the Earth has moved about one degree around its orbit; the Earth needs roughly 4 more minutes of spin to catch the Sun up. Over a year these 4-minute differences add up to exactly one extra sidereal day compared with solar days. The values 23 h 56 min and 24 h are on the data sheet.

Sidereal versus synodic months

The same logic that makes the solar day longer than the sidereal day makes the synodic month longer than the sidereal month. The phase cycle you actually see (full Moon to full Moon) is the 29.5 day synodic month, while the 27.3 day sidereal month is the true orbital period that matches the Moon's rotation (Topic 2). Keeping these two figures apart is frequently examined.

The lunar phase cycle

The Moon makes no light of its own; half of it is always sunlit, and the phase is just how much of that lit half we can see. New Moon happens when the Moon is roughly between Earth and Sun (its dark side towards us); full Moon when it is opposite the Sun (its lit side towards us). The phases also set when eclipses are possible (solar at new, lunar at full).

Equinoxes and solstices

The tilt, not the changing Earth-Sun distance, drives the seasons and the Sun's varying path. At the June solstice the northern hemisphere tilts towards the Sun (long days, high noon Sun); at the December solstice it tilts away. The equinoxes mark the changeover, when the Sun is overhead at the equator. These points anchor the calendar and the alignment of many ancient monuments (Topic 7).

How Edexcel examines this

This is naked-eye Paper 1 content rich in explanation marks. The sidereal versus solar day question is common: define each (relative to the stars versus the Sun), give the values from the data sheet, and explain the extra 4 minutes by the Earth's orbital motion. The sidereal versus synodic month uses the same reasoning with the Moon, and you must keep 27.3 days (sidereal) and 29.5 days (synodic) distinct. Phase questions reward the changing view of the sunlit half and identifying new and full Moon by the Sun-Earth-Moon geometry, often linked to eclipses. Equinox and solstice questions reward attributing the changing day length and Sun path to the $23.5$ degree tilt. The most penalised errors are swapping the day or month lengths and saying the Moon emits light, so anchor sidereal to the stars and remember the Moon only reflects.

Try this

Q1. State which is longer, the sidereal day or the solar day, and by roughly how much. [1 mark]

• Cue. The solar day is longer by about 4 minutes.

Q2. State what causes the phases of the Moon. [1 mark]

• Cue. Our changing view of the Moon's sunlit half as it orbits the Earth.