Plate 05 · Space
Why Summer Is Warm
Not because the Earth is closer to the Sun. In early January the Earth is at its closest — in the dead of the northern winter. The answer lies in a tilt.
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A Tilted Axis
As the Earth travels around the Sun, it spins on its axis. That axis is not perpendicular to the plane of its orbit; it leans about 23.4 degrees from upright.
More important, it points the same way wherever the Earth is in its orbit. So over a year, the Northern Hemisphere is sometimes tipped toward the Sun and sometimes away from it.
Sizes and distances of Sun and Earth not to scale · the tilt is the true angle
June: The North Leans In
In June the Northern Hemisphere leans toward the Sun. At the solstice the Sun climbs higher and stays up longer: in Beijing, daylight lasts 15 hours.
On the same day the Southern Hemisphere leans away. In Sydney the day is under 10 hours long, and it is winter.
The Same Beam of Sunlight
When the Sun is high, its light comes down steeply. When it is low, the same beam strikes at a slant and spreads over more ground, so each patch of ground gets less of its heat.
In Beijing the noon Sun stands about 73 degrees high at the June solstice and only about 27 degrees in December. In winter the same beam is spread over more than twice the area — and the days are far shorter, too.
Left: noon at the June solstice · right: noon at the December solstice · Beijing
Not Because It Is Closer
The Earth’s orbit is very nearly a circle. In early January the Earth is closest to the Sun, about 147 million kilometers away; in early July it is farthest, about 152 million — a difference of only about 3 percent.
And January, when we are closest, is the coldest time of year in the north. If distance made the seasons, both hemispheres would have summer at once. In fact their seasons are opposite.
Shape of the orbit to scale · Earth and Sun enlarged
The Sun That Never Sets
The closer to the poles, the more extreme the seasons. Inside the Arctic Circle there are summer days when the Sun never sets, and winter days when it never rises.
In Tromsø, in northern Norway, the Sun stays up from late May to late July. From late November to mid-January it does not rise at all.
A Year in Four Cities
Plot the length of every day of the year. The closer to the equator, the flatter the line: in Singapore the day is about 12 hours long all year. Sydney’s curve roughly mirrors Beijing’s, upside down and a little flatter: it lies in the Southern Hemisphere, and a little nearer the equator.
Day lengths computed day by day with the Astronomical Almanac formulas
Your Turn
Drag the date and watch the sunlight move across the Earth. Choose another city and watch the noon Sun height and the area one beam covers change in the readouts. Then stand the axis upright and see what a world without a tilt would be like.
Try thisSet the tilt of the axis to 0°: how long is a June day in Tromsø?
What the model leaves out
The sizes of the Sun and Earth and the distance between them are not to scale; only the tilt of the axis and the shape of the orbit are true. How warm a season is also depends on oceans, air and terrain; this plate deals only with the angle of the sunlight and the length of the day. Sunrise, sunset and the noon Sun are computed with the Astronomical Almanac’s low-precision formulas, accurate to a minute or two.
A common misreading
Summer is not warm because the Earth is closer to the Sun. The Earth is closest in early January, when it is winter in the north and summer in the south. The seasons come from the tilt of the axis, which turns each hemisphere toward the Sun in turn.
Sources
- NASA Space Place: What causes the seasons?
- NASA Astronomy Picture of the Day: perihelion (January 2025)
- NASA: Earth fact sheet (axial tilt)
- NOAA Solar Calculator: how sunrise, sunset and Sun height are computed
Data
Earth: NASA Earth Observatory, Blue Marble Next Generation, October 2004 (Reto Stöckli); city lights, NASA Earth Observatory Black Marble 2016 (Joshua Stevens, Miguel Román, Suomi NPP VIIRS); clouds, NASA Blue Marble 2002 (Reto Stöckli, NASA GSFC). Elevation (globe relief): Terrain Tiles on AWS Open Data; ETOPO1, NOAA; GMTED2010 and SRTM, USGS. Stars: Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren 1991), via CDS VizieR. Milky Way: NASA/Goddard Space Flight Center Scientific Visualization Studio, Deep Star Maps 2020; Gaia DR2: ESA/Gaia/DPAC.
Follow the question
Plate 12Why the Moon Changes ShapeThe Moon never changes shape. What changes is how much of its sunlit half we can see.
Plate 03The Shadow of the MoonThe Sun is about 400 times wider than the Moon, and about 400 times farther away. That coincidence lets day turn to night for a few minutes.
