The Earth's axis is inclined to the plane of its orbit (the plane of the ecliptic) at an angle of about:

The Earth's axis makes an angle of about 66.5 degrees (66 degrees 30 minutes) with the plane of the Earth's orbit around the Sun, called the plane of the ecliptic.

Two ways of describing the same tilt:

• The axis is tilted about 23.5 degrees from the perpendicular (a line at right angles) to the orbital plane.
• Since 90 minus 23.5 = 66.5, the axis makes an angle of about 66.5 degrees with the orbital plane itself.
• Both statements describe the same tilt; exam questions specify which angle they mean.

Precise value:

The present tilt (obliquity) is about 23.44 degrees from the perpendicular.

Effects of the tilt:

• Seasons: As the Earth revolves, each hemisphere is tilted towards the Sun for part of the year (summer) and away from it for the other part (winter).
• Changing length of day and night: Except at the Equator, the length of day and night changes through the year.

Position of the Tropics: The Tropic of Cancer and the Tropic of Capricorn lie at about 23.5 degrees North and South because of the tilt. These are the farthest latitudes where the Sun can be directly overhead.

Position of the polar circles: The Arctic and Antarctic Circles lie at about 66.5 degrees North and South. Beyond them, there is at least one day of continuous daylight in summer and one day of continuous night in winter.

Without the tilt:

If the Earth's axis were perpendicular to its orbit, there would be no seasons, and day and night would be equal everywhere throughout the year.

Why not the other options?

• ✗23.5 degrees is the tilt measured from the perpendicular, not from the orbital plane.
• ✗45 degrees and 90 degrees are incorrect; at 90 degrees, the axis would be perpendicular to the orbit.