Phases, Seasons and Time Zones

The Lunar Phase Cycle

The Moon does not produce its own light. The phases we see are simply different proportions of the Moon’s sunlit hemisphere facing Earth as the Moon orbits us. To understand phases, imagine looking down on the Earth–Moon system from above the North Pole: the Moon orbits anticlockwise, and the Sun illuminates half the Moon at any time. As the Moon’s position changes relative to Earth and Sun, we see different fractions of that illuminated half.

The eight named phases, in order, are:

• New Moon — Moon is between Earth and Sun; the near side faces away from sunlight; the Moon is not visible.
• Waxing Crescent — a thin sliver of lit surface becomes visible in the west after sunset.
• First Quarter — the Moon is 90° east of the Sun; the right half of the near side is lit; rises around midday and sets around midnight.
• Waxing Gibbous — more than half the near side is lit, still growing (‘waxing’).
• Full Moon — Earth is between Moon and Sun; the entire near side is lit; rises at sunset, sets at sunrise.
• Waning Gibbous — the lit fraction is decreasing (‘waning’).
• Last Quarter — the left half of the near side is lit; rises around midnight and sets around midday.
• Waning Crescent — a thin sliver visible in the east before sunrise; completes the cycle back to new moon.

Sidereal Month vs Synodic Month

Just as there are two types of day, there are two types of month. The sidereal month is the time for the Moon to orbit Earth relative to the distant stars: approximately 27.3 days. However, because Earth is simultaneously moving around the Sun, the Moon must travel a little further along its orbit to re-align with the Sun and return to the same phase. This longer period — 29.5 days — is the synodic month, sometimes called the lunar month. It is the synodic month that governs the visible phase cycle and is used for lunar calendars.

The difference between 27.3 and 29.5 days arises for exactly the same geometrical reason as the difference between a sidereal and solar day: Earth’s orbital motion means the Moon must ‘catch up’ an additional ~27° of arc before the Sun, Earth and Moon return to the same relative geometry.

Seasons: Axial Tilt, Not Distance

One of the most persistent misconceptions in science education is that seasons are caused by Earth getting closer to or further from the Sun. This is wrong for a simple reason: Earth is actually closest to the Sun in early January (perihelion, ~147 million km) — which is winter in the Northern Hemisphere. Distance to the Sun plays almost no role in seasons.

Seasons are caused by Earth’s axial tilt of 23.5°. Because the axis always points in the same direction in space (toward Polaris), different hemispheres are tilted toward or away from the Sun at different times of year:

• At the June solstice (around 21 June), the Northern Hemisphere tilts toward the Sun. Sunlight arrives at a steeper angle, is more concentrated per unit area, and the days are longer — producing summer in the North.
• At the December solstice (around 21 December), the Southern Hemisphere tilts toward the Sun. The Northern Hemisphere has longer nights and shallower, more spread-out sunlight — producing winter.
• At the two equinoxes (March and September), neither hemisphere is tilted toward the Sun, and day and night are of roughly equal length worldwide.

The variation in sunrise and sunset time throughout the year follows directly from the axial tilt: at the summer solstice in the UK, the Sun rises as early as about 04:45 BST (in June), while at the winter solstice it may not rise until after 08:00 GMT. Observers at or above the Arctic Circle (66.5° N) experience the midnight Sun around the June solstice — 24 hours of daylight.

Time Zones and Greenwich Mean Time

If every location kept its own Local Mean Time, clocks would differ from one town to the next, making international travel and communication chaotic. The solution, adopted from the late nineteenth century, is to divide the world into time zones: bands roughly 15° of longitude wide (since Earth rotates 15° per hour), each sharing a standard clock time offset from Greenwich Mean Time (GMT) by a whole number of hours.

GMT is the Local Mean Time at the Prime Meridian (0° longitude, Greenwich). It serves as the international reference from which all other time zones are offset. Universal Time (UT) is the modern name for the same scale, set by Earth’s rotation: GMT and UT mean the same time.

The relationship between longitude and time is precise: Earth rotates 360° in 24 hours, so it rotates 15° per hour or 1° per 4 minutes. A location 30° east of Greenwich is therefore 2 hours ahead of GMT; a location 75° west of Greenwich is 5 hours behind GMT. Time zones allow a location to keep clock noon near solar noon, within ±30 minutes for most standard zones.