Why Seasons Happen: Axial Tilt Explained

Why Seasons Happen: Axial Tilt Explained

Seasons come from Earth's 23.44° tilt, not its distance from the Sun, and near 6-7° N that tilt shows up as an overhead Sun and wet and dry seasons.

Related: Finding Objects in the Night Sky Without Polaris · Sun Path Today, Hour by Hour · Sun/Moon Times & Dark Window

All times on this page are Guyana time (America/Guyana, UTC-4, no daylight saving). Georgetown numbers use 6.80° N, 58.16° W. Lethem numbers use 3.38° N, 59.80° W.

The short answer

Earth's axis is tilted 23.44° from a line straight up out of its orbit. The axis keeps pointing the same way in space all year. So for half the year the northern half of Earth leans toward the Sun, and for the other half the southern half does. The half that leans toward the Sun gets a higher noon Sun and longer days, so it warms up. That is summer.

Distance is not the cause. Earth is closest to the Sun in early January, in the middle of northern winter.

In Guyana, at about 6-7° N, the tilt still works, but the effect on temperature is small. What you notice instead is the Sun passing straight overhead twice a year, and wet and dry seasons set by a moving rain belt.

Earth's two motions

Earth's spin gives us the day. Its trip around the Sun gives us the year.

Rotation: the day

Earth turns from west to east. That is why the Sun, Moon and stars rise in the east and set in the west.

One full turn measured against the distant stars takes 23 h 56 m 04 s. This is the sidereal day. A solar day, from one noon to the next, averages 24 hours. The solar day is longer because Earth moves about 1° along its orbit each day, so it must turn a little extra to bring the Sun back to the same place.

You can see the difference at night: each star rises about 4 minutes earlier than the night before.

One turn against the stars and one noon to the next: why the solar day runs about four minutes longer.

Earth's spin is not perfectly steady. Tides slow it very slightly, and changes inside the Earth speed it up or slow it down by tiny amounts. To keep clock time in step, 27 leap seconds were added between 1972 and 2016. None has been added since, and an international decision in 2022 phases them out by 2035.

Time zones and Guyana Time

World time is based on UTC (Coordinated Universal Time). In theory each time zone is 15° of longitude wide, because Earth turns 15° an hour. Real zones bend to follow borders.

Guyana uses Guyana Time, UTC-4, with no daylight saving. That offset matches the 60° W meridian. Georgetown sits at about 58.2° W, a little east of 60° W, so on average the Sun reaches its highest point about 7 minutes before 12:00. Earth's tilted, oval orbit then shifts solar noon through the year. At Georgetown it falls between about 11:36 (early November) and 12:07 (mid-February).

Orbit: the year

Earth takes 365.2422 days to go from one March equinox to the next. This is the tropical year, and it is the year that keeps step with the seasons.

The orbit is a slightly stretched circle, with an eccentricity of 0.017.

| Point in the orbit | When | Distance from the Sun | Orbital speed | |---|---|---|---| | Perihelion (closest) | early January (2-5 Jan) | 147.1 million km | 30.29 km/s | | Aphelion (farthest) | early July (3-6 Jul) | 152.1 million km | 29.29 km/s |

The distance changes by only 3.4% over the year. Earth's average speed is 29.78 km/s. It moves fastest when closest to the Sun, as Kepler's second law says.

Why seasons happen: tilt, not distance

The tilt changes two things for any place on Earth.

At mid-latitudes both effects are large, and together they make hot summers and cold winters. Four dates mark the cycle.

At the equinoxes, day and night are nearly equal everywhere. Daylight still runs a few minutes over 12 hours, because the air bends sunlight over the horizon and sunrise is counted from the top edge of the Sun. At Georgetown on 20 March 2026 the Sun rises at 05:57 and sets at 18:03, about 12 h 07 min of daylight.

Here is the same tilt seen from three places.

| | Georgetown (6.8° N) | Lethem (3.4° N) | 45° N | |---|---|---|---| | Noon Sun, June solstice | 73.4° up, 16.6° north of overhead | 69.9° up, 20.1° north of overhead | 68.4° up, in the south | | Noon Sun, December solstice | 59.8° up, 30.2° south of overhead | 63.2° up, 26.8° south of overhead | 21.6° up, in the south | | Daylight, 21 June | 12 h 31 min | 12 h 19 min | 15 h 37 min | | Daylight, 21 December | 11 h 44 min | 11 h 56 min | 8 h 46 min | | Change over the year | 47 min | 23 min | about 6 h 50 min |

Drag through the year: Earth's tilted axis on its orbit, and the noon Sun and daylight it gives Georgetown, Lethem and 45° N. Guyana's noon Sun swings a lot while its daylight hardly moves; at 45° N both change.

At 45° N the noon Sun swings from high to low and the days nearly double in length. In Guyana the noon Sun still swings across the sky, but the days barely change.

What seasons mean in Guyana

The Sun passes overhead twice a year

Anywhere between the two tropics, the noon Sun is straight overhead on two days a year. On those days a vertical pole casts almost no shadow at noon.

Between those dates the noon Sun is north of overhead. At Georgetown that runs from about 7 April to 4 September (29 March to 13 September at Lethem). The rest of the year the noon Sun is in the south. Two overhead Suns do not make two summers. The temperature change they bring is small.

The noon Sun's distance from overhead through the year from Georgetown and Lethem, with the two days it passes straight overhead and the earliest and latest solar noon.

Every noon of 2027 over Georgetown and London: in Georgetown the Sun passes overhead in early April and early September, and the noon shadow flips sides; in London the noon Sun only ever swings north and south of the same southern sky.

Day length barely changes

At Georgetown the longest day (21 June) is only 47 minutes longer than the shortest (21 December): 12.52 hours against 11.73 hours. At Lethem, closer to the equator, the gap is 23 minutes. Sunrise stays between about 05:35 and 06:11 all year. Check any date yourself in Sun/Moon Times & Dark Window, which lists sunrise and sunset for the date you pick.

Daylight through the year at Georgetown, Lethem, 45° N and London.

Guyana's real seasons are wet and dry

Because sunshine changes so little, Guyana's temperatures stay fairly steady through the year. The seasons people feel are rainfall seasons.

They come from the Intertropical Convergence Zone (ITCZ). This is a band of showers and thunderstorms near the equator, where the northeast and southeast trade winds meet and push air upward. The ITCZ follows the Sun: it moves north during the northern summer and south during the northern winter. As it passes over Guyana it brings rain.

So the tilt still drives Guyana's seasons, just indirectly: it moves the Sun, and the Sun moves the rain belt.

Month by month

| Months | Rain on the coast | Noon Sun at Georgetown | Evening sky (about 20:00-22:00) | |---|---|---|---| | December to February | Wet to the end of January, dry from February. The south is dry. | South of overhead, lowest near 21 December | Orion high. Betelgeuse passes almost overhead near 22:00 in mid-January. | | March to May | Dry in March, main wet season from mid-April | Moving north, overhead about 6 April | Leo and Virgo high. Regulus is highest near 20:25 in mid-April. | | June to August | Wet to the end of July, drying out from August | North of overhead until 4 September | Milky Way core high from July, 54° up in the south near 22:00 in mid-July. | | September to November | Dry to the end of October, wet again from mid-November | Overhead about 4 September, then moving south | Pegasus and Andromeda. September and October fall in the coast's long dry season. |

For the Milky Way months in detail, read Milky Way Season Near the Equator and try the Milky Way Season Planner. For working through the wet seasons, see the Dew and Monsoon Stargazing Playbook.

Why the night sky changes through the year

The 4-minute daily shift adds up to about 2 hours a month. A star that is highest at 22:00 in mid-January is highest at about 20:00 in mid-February, and months later it is up only in daylight. That is why each season has its own evening constellations.

The planets move against this background of stars, which is why the ancient Greeks called them "wanderers". To find stars and planets by their sky coordinates, see Finding Objects in the Night Sky Without Polaris.

From Guyana you see much of both hemispheres, and targets on the celestial equator pass nearly overhead, through less air than from farther north.

Calendars: keeping step with the seasons

A calendar year has a whole number of days, but the tropical year is 365.2422 days. Leap years make up the difference.

The Gregorian leap-year rules:

Slow changes: precession and more

Earth's axis wobbles slowly, like a spinning top. One wobble takes about 26,000 years. This is precession. It changes which star sits near the celestial pole, so Polaris will not always be the pole star. It also slowly shifts every star's sky coordinates, which is why star charts name a reference date such as J2000.0.

The tilt itself changes too. Over the last million years it has varied between about 22.1° and 24.5°, in a cycle of about 41,000 years. The shape of the orbit also changes over about 100,000 years. Together these slow cycles (Milankovitch cycles) are linked to the timing of ice ages.

Try it: shadow stick and zero-shadow noon

Sun safety: never look at the Sun, with or without binoculars, a camera or sunglasses. Watch the shadow on the ground, not the Sun.

  1. Stand a straight stick or pole exactly upright on flat, open ground. A 1 m stick makes the numbers easy.
  2. From about 11:30 to 12:15, mark the tip of the shadow every 5 minutes.
  3. The shortest shadow marks local solar noon. Write down the time.
  4. Note which way the noon shadow points and measure its length.
  5. Repeat once a week or once a month.

What to expect at Georgetown with a 1 m stick:

| Date | Noon shadow | |---|---| | About 21 December | 0.58 m, pointing north | | About 20 March | 0.12 m, pointing north | | About 6 April | almost none (Sun overhead) | | About 21 June | 0.30 m, pointing south | | About 4 September | almost none (Sun overhead) |

At an equinox the angle between the stick and the line from the shadow tip to the stick top equals your latitude. At Georgetown that is about 7°.

The shadow stick on any day of the year: its noon shadow from Georgetown or Lethem, and the shadow tip every five minutes from 11:30 to 12:15.

To see the Sun's height and direction hour by hour for today at your location, open Sun Path Today, Hour by Hour. For morning and evening light times, use the Golden & Blue Hour Timeline.

Star version: pick a bright star that sets behind a rooftop or tree. Note the time it disappears on several nights from the same spot. It will vanish about 4 minutes earlier each night.

Year-long version: photograph the Sun's position from a fixed spot at the same clock time every week, or leave a pinhole can out for months. The Sun traces a figure-8 (the analemma) or a band of daily arcs (a solargraph). Solargraphy: Long-Exposure Sun Paths on Paper shows how, and which way to aim a can from Guyana so the arcs stay on the paper all year. The Solargraph Install Planner and the Solar Pinhole Camera help you plan the install.

Common mix-ups

"Seasons happen because Earth gets closer to the Sun." No. Earth is closest in early January, during northern winter. The tilt causes the seasons.

"Two overhead Suns mean two summers." No. Guyana's seasons are wet and dry seasons, set by the ITCZ.

"A day is exactly 24 hours." The mean solar day is 24 hours. One turn against the stars takes 23 h 56 m 04 s, and Earth's spin rate varies slightly.

Quick check

Earth is closest to the Sun in early January. Why is January not summer in the Northern Hemisphere?

Because seasons come from the 23.44° tilt. In January the Northern Hemisphere leans away from the Sun, so it gets a low noon Sun and short days.

On about which dates is the noon Sun straight overhead at Georgetown?

About 6 April and 4 September.

Which way does a noon shadow point at Georgetown in June?

South, because the noon Sun is north of overhead from about 7 April to 4 September.

How much does day length change over the year at Georgetown?

About 47 minutes, from 11.73 hours on 21 December to 12.52 hours on 21 June.

Was 1900 a leap year?

No. It is a century year not divisible by 400. 2000 was a leap year.

Where next

Continue with ASTRO-104: The Moon and Lunar Phenomena, which builds on Earth's orbit to explain phases, tides and eclipses.

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