The term perihelion refers to the point in the orbit of a planet asteroid or comet where it is closest to the Sun. This concept is essential in astronomy and celestial mechanics as it affects the movement speed and climatic conditions of planetary bodies.
In this topic we will explore the definition of perihelion its opposite (aphelion) its impact on Earth and its significance in space science.
Definition of Perihelion
Perihelion comes from the Greek words “peri-“ (meaning near) and “helios” (meaning Sun). It is the moment in an object’s elliptical orbit when it reaches its closest distance to the Sun.
For example:
- Earth reaches its perihelion around January 3rd each year.
- At this point Earth is about 147 million kilometers (91 million miles) from the Sun.
Opposite of Perihelion: Aphelion
The term aphelion refers to the farthest point in an object’s orbit from the Sun. For Earth this occurs around July 4th when the distance is about 152 million kilometers (94 million miles).
Why Do Orbits Have a Perihelion and Aphelion?
Most planets and celestial bodies follow an elliptical (oval-shaped) orbit rather than a perfect circle. This is due to:
- Gravitational forces between the Sun and planets.
- Variations in orbital speed caused by Kepler’s Laws of Planetary Motion.
Kepler’s Second Law and Perihelion
According to Kepler’s Second Law (the Law of Equal Areas):
- A planet moves faster at perihelion because the Sun’s gravity pulls it more strongly.
- It moves slower at aphelion due to the weaker gravitational pull.
The Impact of Perihelion on Earth
1. Does Perihelion Affect Seasons?
Many people assume that perihelion causes summer but this is incorrect. Seasons are determined by Earth’s axial tilt not by its distance from the Sun.
- When Earth is at perihelion (January) it is winter in the Northern Hemisphere and summer in the Southern Hemisphere.
- When Earth is at aphelion (July) the situation is reversed.
2. Temperature Changes Due to Perihelion
Although Earth is closest to the Sun at perihelion the extra solar radiation it receives is not enough to drastically change global temperatures. The difference in solar energy is only about 7% more than at aphelion.
3. Effect on Earth’s Orbital Speed
During perihelion Earth’s orbital speed increases to about 30.29 km/s (compared to 29.29 km/s at aphelion). This means:
- Winters in the Northern Hemisphere are slightly shorter.
- Summers in the Southern Hemisphere are shorter but more intense.
Perihelion in Other Planets
Every planet has a different perihelion depending on its distance from the Sun and orbital eccentricity (how stretched its orbit is).
Perihelion Distances of Some Planets:
| Planet | Perihelion Distance (Million km) | Aphelion Distance (Million km) |
|---|---|---|
| Mercury | 46.0 | 69.8 |
| Venus | 107.5 | 109.3 |
| Earth | 147.1 | 152.1 |
| Mars | 206.7 | 249.2 |
| Jupiter | 740.6 | 816.6 |
Mercury’s Extreme Perihelion
Mercury has the most eccentric orbit of any planet meaning its perihelion distance is much smaller than its aphelion distance. This leads to extreme temperature variations on its surface.
Perihelion and Space Science
1. Studying Climate and Orbital Changes
Over long periods Earth’s perihelion gradually shifts due to gravitational interactions with other planets. This slow change is part of Milankovitch Cycles which influence climate patterns and ice ages.
2. Effect on Satellites and Space Missions
Space agencies calculate perihelion and aphelion carefully when planning satellite orbits and interplanetary missions. Understanding perihelion helps in:
- Optimizing fuel efficiency for spacecraft.
- Timing space probes’ close approaches to the Sun (like NASA’s Parker Solar Probe).
Perihelion is the point in an orbit where a celestial body is closest to the Sun. While it affects orbital speed and solar radiation levels it does not directly cause seasons. The study of perihelion is crucial in astronomy climate science and space exploration.
By understanding perihelion and its effects we gain deeper insights into planetary motion climate variations and the dynamics of our solar system.
