The Earth’s orbit around the Sun is not a perfect circle but an elliptical path. This means that our planet is sometimes closer to the Sun and sometimes farther away. These two points in Earth’s orbit are known as perihelion and aphelion.
Understanding these terms is essential for learning about Earth’s movement, seasons, and how distance from the Sun affects our planet. This topic explains the differences between perihelion and aphelion, their effects, and why they occur.
What Is Perihelion?
Perihelion is the point in Earth’s orbit when it is closest to the Sun. The word comes from Greek, where “peri-” means “near” and “helios” means “Sun”.
When Does Perihelion Occur?
Perihelion occurs every year around early January (usually between January 2nd and 5th). At this time, Earth is about 147 million kilometers (91 million miles) from the Sun.
Effects of Perihelion
- Increased Solar Radiation – Earth receives slightly more solar energy due to its proximity to the Sun.
- Higher Orbital Speed – According to Kepler’s second law, Earth moves faster in its orbit at perihelion.
- No Major Effect on Seasons – Perihelion does not cause summer in the Northern Hemisphere because seasons are influenced by Earth’s axial tilt, not distance from the Sun.
What Is Aphelion?
Aphelion is the point in Earth’s orbit when it is farthest from the Sun. The term comes from Greek, where “apo-” means “away” and “helios” means “Sun”.
When Does Aphelion Occur?
Aphelion happens every year around early July (usually between July 3rd and 7th). At this time, Earth is about 152 million kilometers (94 million miles) from the Sun.
Effects of Aphelion
- Slightly Less Solar Radiation – Earth receives about 7% less sunlight than at perihelion.
- Lower Orbital Speed – Earth moves slower in its orbit at aphelion.
- No Major Effect on Seasons – Despite being farther from the Sun, the Northern Hemisphere experiences summer because of Earth’s axial tilt.
Key Differences Between Perihelion and Aphelion
| Feature | Perihelion | Aphelion |
|---|---|---|
| Definition | Closest point to the Sun | Farthest point from the Sun |
| Distance from Sun | ~147 million km (91 million miles) | ~152 million km (94 million miles) |
| Time of Year | Early January | Early July |
| Orbital Speed | Faster | Slower |
| Solar Radiation | Slightly higher | Slightly lower |
| Effect on Seasons | Minimal | Minimal |
Why Does Earth’s Distance from the Sun Change?
Earth follows an elliptical orbit, which is described by Kepler’s Laws of Planetary Motion:
- First Law (Elliptical Orbits) – Planets move in an ellipse, with the Sun at one focus of the ellipse.
- Second Law (Equal Areas in Equal Time) – Earth moves faster at perihelion and slower at aphelion to maintain a constant area sweep in equal time.
- Third Law (Orbital Periods and Distance) – The farther a planet is from the Sun, the longer its orbital period.
Does Perihelion Make Winters Warmer?
You might think that being closer to the Sun at perihelion would make winters warmer. However, Earth’s seasons are not caused by distance from the Sun but by its axial tilt.
- The Northern Hemisphere’s winter occurs around perihelion because the North Pole is tilted away from the Sun.
- The Southern Hemisphere’s summer occurs during perihelion, making summers in the Southern Hemisphere slightly warmer than those in the Northern Hemisphere.
How Perihelion and Aphelion Affect Earth’s Climate
Although the difference in distance between perihelion and aphelion is small, it has minor effects on Earth’s climate:
- Seasonal Differences Between Hemispheres – Because perihelion occurs during Southern Hemisphere summer, this hemisphere experiences warmer summers and cooler winters.
- Length of Seasons – Since Earth moves faster at perihelion and slower at aphelion, the Northern Hemisphere’s winter is shorter, while its summer is slightly longer.
Perihelion and Aphelion on Other Planets
Other planets also experience perihelion and aphelion, but their effects can be more extreme due to more eccentric (oval-shaped) orbits.
| Planet | Perihelion Distance | Aphelion Distance | Effect on Climate |
|---|---|---|---|
| Mercury | 46 million km | 70 million km | Huge temperature swings |
| Mars | 207 million km | 249 million km | More extreme seasons |
| Jupiter | 741 million km | 817 million km | Minimal impact |
| Pluto | 30 AU | 49 AU | Drastic climate changes |
Will Earth’s Orbit Change Over Time?
Yes, Earth’s orbit is not fixed. Due to gravitational influences from other planets, particularly Jupiter and Saturn, Earth’s orbit undergoes slow changes over thousands of years.
This is part of the Milankovitch Cycles, which influence Earth’s climate and ice ages.
Perihelion and aphelion are key points in Earth’s orbit, representing its closest and farthest distances from the Sun.
- Perihelion occurs in January when Earth is closest to the Sun and moves faster.
- Aphelion happens in July, when Earth is farthest from the Sun and moves slower.
- These points do not cause seasons but slightly influence climate and orbital speed.
Understanding perihelion and aphelion helps us grasp Earth’s motion and its impact on our planet’s climate and history.
