What are Perihelion and Aphelion?

If the sun appears a bit more intense than normal to you this week, you have a keen eye. The Earth has just made its closest approach to our nearest star for the year.

The orbital milestone is known as “perihelion,” and it marks the time when the distance between the Earth and the Sun is at its smallest. The event occurs every year in early January, and in 2019 it took place Thursday, Jan. 3 at 12:20 a.m. EST (or at 05:20 GMT, depending on your time zone).

On average, the Earth orbits the Sun at a distance of about 93 million miles (150 million kilometers). This distance is known as 1 astronomical unit (AU), and it serves as a yardstick for distances to other planets in our Solar System. Mars, for example, is about 1.5 AU from the Sun, while Jupiter is about 5.2 AU from the star.

But like the other Planets in our Solar System, the Earth’s orbit is not a perfect circle. Instead, it is slightly elliptical — or oval-shaped — meaning it has a closest point to the sun (perihelion) and a farthest point (which is known as aphelion).

During the perihelion, the Earth is about 91.3 million miles (147 million km) from the sun, or about 0.983 AU. The Earth will reach aphelion on July 5. At that time, our planet will be about 94.5 million miles (152 million km) — or 1.017 AU — from the sun.

The difference between the two extremes of Earth’s orbit is just over 3 million miles (5 million km). In January, the sun can appear to shine about 7 percent more intensely than it does in July during aphelion, according to NASA descriptions.

If you live in the Northern Hemisphere, the fact that Earth is closest to the sun during the cold winter season may be confusing, but there is an explanation.

The Earth’s changing seasons are actually determined by our planet’s tilt on its axis, not its distance from the sun. Our planet spins on an axis that is tilted about 23.5 degrees from vertical. This tilts the Northern Hemisphere away from the sun during the northern winter and toward the sun during the northern summer.

 

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