Know that the Earth orbits the Sun once in approximately 365 days and use this to explain the periodic nature of the seasons

6.1.1 The Earth – The Grand Tour Around the Sun 🌍✈️

Orbit Basics

Think of the Earth as a glittering dancer twirling around the Sun. The path it follows is called an orbit, and it’s not a perfect circle but a slightly flattened oval (an ellipse). The Sun sits near one focus of this ellipse, so the Earth is a bit closer at one point (perihelion) and a bit farther at another (aphelion).

The time taken for one complete orbit is called the sidereal year and is approximately

\$T \approx 365.25\ \text{days}.\$

Because of this, the Earth completes a full circle around the Sun roughly every 365 days.

Why 365 Days?

• Our calendar is based on the Earth’s orbit, so one year ≈ 365 days.
• Every four years we add a leap day (Feb 29) to keep the calendar in sync with the orbit.
• Mathematically, the average orbital period is

  \$\overline{T} = \frac{365\ \text{days} + 1\ \text{day}}{4} = 365.25\ \text{days}.\$

Seasonal Changes

Seasons are caused by the Earth’s tilt (about \$23.5^\circ\$) relative to its orbital plane. As the Earth orbits, different hemispheres point toward or away from the Sun, changing the intensity and duration of sunlight.

Key Terms

1. Orbit – the path an object takes around another.
2. Sidereal year – time for one full orbit relative to distant stars.
3. Perihelion – closest point to the Sun.
4. Aphelion – farthest point from the Sun.
5. Axial tilt – angle of Earth’s axis relative to its orbital plane.

Quick Quiz

1️⃣ What is the approximate length of a sidereal year?

2️⃣ Why do we have a leap year every four years?

3️⃣ Which factor mainly causes the seasons to change?

4️⃣ At what point in its orbit is the Earth closest to the Sun?

5️⃣ If the Earth’s tilt were 0°, what would happen to the seasons?