Earth's Axial Tilt: The 23.4-Degree Lean That Gives Us Seasons

Earth does not spin upright. Its axis leans, and that lean is the reason a year has seasons. The lean is modest, yet it decides how high the noon Sun climbs, how long the day lasts and where on Earth the Sun can ever shine straight down.

This guide gives NASA's value for the tilt and shows why other official pages print slightly different numbers. It explains what the tilt does, and why Earth's distance from the Sun does not. It ends with a task in the Orbit Dial: finding the latitude where the Sun is overhead at noon on the June solstice.

What the tilt is

NASA's Earth Facts page says Earth's axis of rotation is tilted 23.4 degrees with respect to the plane of Earth's orbit around the Sun. The U.S. Naval Observatory calls this angle the obliquity and says the seasons are caused by it. NOAA words it a different way. Instead of rotating with its axis perpendicular to its orbital plane, Earth is tilted 23.45 degrees from the perpendicular.

The Naval Observatory adds a thought experiment. If Earth's axis were perpendicular to its orbital plane, there would be no seasons at all.

The tilt also stays pointed in one direction. NASA's Space Place says that as Earth orbits the Sun, its tilted axis always points in the same direction. So the North Pole tilts toward the Sun around June, and the South Pole tilts toward it around December.

The drawing above shows the June solstice for a place at 23.4 degrees north latitude.

One angle, several roundings

If you check official pages, you will find the tilt printed as 23.4, 23.44, 23.45 and 23.5 degrees. These are one angle, written to different decimal places or rounded in different ways. The table lists what each source prints.

This site uses about 23.4 degrees, the value on NASA's Earth Facts page and the Naval Observatory's seasons page. The angle also drifts over very long spans. The Naval Observatory describes a 41,000-year variation in obliquity, so the figure is a present-day value, not a permanent one.

What the tilt does

Because the axis keeps its direction, each hemisphere takes turns leaning toward the Sun. NASA's Earth Facts page says that when the Northern Hemisphere leans toward the Sun it gets more direct sunlight and has summer, while the Southern Hemisphere has winter. Six months later it flips. NOAA adds that different parts of Earth receive the Sun's direct rays at different times of year.

The Naval Observatory defines the solstices as the two dates when the axis is most tilted toward or away from the Sun. At the equinoxes it is tilted toward neither. You can see those two cases in our guides to the summer solstice and the spring equinox.

The tilt also fixes where the Sun can stand straight overhead. NASA's Earth Observatory says that at the June solstice the Sun is above the Tropic of Cancer, and at the December solstice it is over the Tropic of Capricorn. NASA's Night Sky Network places those lines 23.5 degrees north and 23.5 degrees south of the equator. The Naval Observatory gives the Sun's declination, its angle north or south of Earth's equator, as +23.4 degrees at the summer solstice and -23.4 degrees at the winter solstice.

Those angles match the tilt. At the solstice, the axis leans toward the Sun by the full tilt angle, so the Sun's direct rays reach that far from the equator and no farther. The tropics are the tilt written as a latitude.

The drawing above shows the December solstice for a place at 23.4 degrees south latitude.

Why distance is not the cause

Many people guess that summer happens when Earth is close to the Sun. The numbers say otherwise. NASA's Space Place gives Earth's closest point, perihelion, as 91,400,000 miles from the Sun and its farthest point, aphelion, as 94,500,000 miles. NASA's Earth Fact Sheet gives the same two distances as 147.095 and 152.100 million kilometers.

The Naval Observatory says the distance at perihelion is only about 3 percent less than at aphelion. It also says perihelion falls in early January, only about two weeks after the December solstice. The Northern Hemisphere has winter when Earth is closest to the Sun, and NASA's Space Place concludes that Earth's distance from the Sun is not the cause of the seasons. Our guide to perihelion and aphelion covers the orbit in more detail.

Common mistakes

• Blaming distance. Perihelion comes in January, in the middle of northern winter. The tilt is what changes the seasons.
• Treating 23.4, 23.44, 23.45 and 23.5 as different tilts. They are one angle, printed with different rounding. Pick one and say which source it comes from.
• Picturing a wobbling axis. NASA says the axis always points in the same direction as Earth orbits. What changes during the year is which hemisphere leans toward the Sun.
• Thinking the Sun moves north and south. NASA's Earth Observatory says it is not the Sun that is moving north or south through the seasons. The cause is Earth's orientation toward the Sun.
• Thinking the Sun is overhead at the equator all year. NASA says observers on the equator get two zero-shadow days, at the equinoxes. On the Tropic of Cancer it comes on the June solstice.

Try it in the instrument

Open the Orbit Dial and set the sky to the latitude where the Sun is directly overhead at noon on the June solstice. The dial takes a date in 2026 and a latitude, and it shows the height of the noon Sun and the length of the day. Use the June solstice date, June 21 in the Naval Observatory's table. The target latitude is about 23.4 degrees north, the tilt in NASA's Earth Facts. The game accepts a band of latitudes, not one exact value, and the band gets narrower from Bronze to Gold.

Use NASA's rule to check yourself. On an equinox, the noon Sun's altitude equals 90 degrees minus your latitude, according to NASA's Radio JOVE training module. At 23.5 degrees north that gives 66.5 degrees. The module says to add or subtract 23.5 degrees at the solstices. In June the Sun's declination is positive, +23.4 degrees by the Naval Observatory, so you add, and 66.5 plus 23.5 is 90. The noon Sun is straight overhead. NASA's Night Sky Network says observers on the Tropic of Cancer get their zero-shadow day on that date.

Now slide the latitude on June 21. North of the Tropic of Cancer the noon Sun drops below 90 degrees. At 40 degrees north it stands a little over 73 degrees high. The peak of 90 degrees sits at the latitude equal to the tilt, and that is the answer the game wants.

Questions people ask

Why is the Tropic of Cancer at 23.5 degrees north?

Because it is the farthest north the Sun can stand overhead, and that limit equals Earth's tilt. NASA's Night Sky Network places the line 23.5 degrees north of the equator. The Naval Observatory gives the Sun's northernmost declination as +23.4 degrees, the same number as the tilt.

Does the tilt ever change?

Very slowly. The Naval Observatory describes a 41,000-year variation in obliquity. Over a year, the axis keeps pointing in the same direction, according to NASA's Space Place.

What would Earth be like with no tilt?

The Naval Observatory says there would be no seasons at all if Earth's axis were perpendicular to its orbital plane. The lean is what turns one year of orbiting into four seasons.

Sources

• Facts About Earth, NASA Science. https://science.nasa.gov/earth/facts/. Opened November 10, 2026.
• The Seasons and the Earth's Orbit, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/seasons_orbit. Opened November 10, 2026.
• Earth Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html. Opened November 10, 2026.
• Why does Earth have Seasons?, NOAA NESDIS, K-12 Education. https://www.nesdis.noaa.gov/about/k-12-education/understanding-our-planet/why-does-earth-have-seasons. Opened November 10, 2026.
• Seeing Equinoxes and Solstices from Space, NASA Science, Earth Observatory. https://science.nasa.gov/earth/earth-observatory/seeing-equinoxes-and-solstices-from-space-52248/. Opened November 10, 2026.
• The Seasons, the Equinox, and the Solstices, National Weather Service, Cleveland, OH (NOAA). https://www.weather.gov/cle/seasons. Opened November 10, 2026.
• Tropical Solstice Shadows, NASA Science, Night Sky Network. https://science.nasa.gov/solar-system/skywatching/night-sky-network/tropical-solstice-shadows/. Opened November 10, 2026.
• Sunrise and Sunset Times Near the Solstices, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/rs_solstices. Opened November 10, 2026.
• What Causes the Seasons?, NASA Space Place. https://spaceplace.nasa.gov/seasons/en/. Opened November 10, 2026.
• Citizen Science Training Modules, Training Module 0.0: Introduction to the Sun, NASA Radio JOVE Project (radiojove.gsfc.nasa.gov). https://radiojove.gsfc.nasa.gov/citizen_science/training/TM0.0_The_Sun_Introduction.pdf. Opened November 9, 2026.
• Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/calculated/seasons?year=2026&tz=0&tz_sign=1&tz_label=true&dst=false&submit=Get+Data. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• Summer SolsticeAt the June solstice, Earth's axis leans the Northern Hemisphere as far toward the Sun as it ever goes, so the noon Sun stands highest there.
• Perihelion and AphelionEarth is closest to the Sun around January 3, in the middle of northern winter.