Of all the astronomical terms for our seasonal events, solstice and equinox are hands-down the most familiar and popular. Meantime perihelion and aphelion are much less familiar to us and are rarely the subjects for casual conversation.
This is probably because civilizations have known and observed solstices and equinoxes since prehistory. Most educated people know about the Earth’s tilt and how that tilt relates to the seasonal changes. Meantime, perihelion and aphelion have had to wait until astronomers were able to understand and explain the true nature of planetary orbits until relatively recent times.
It used to be thought that Earth and the other planets orbited the Sun in perfect circles. However, the planets in the Solar System actually have eccentric or elliptical orbits. That means that a planet will sometimes be very near the Sun and the other half of the year it will be at its most distant location from the Sun.
The names for these orbital locations are derived from the Greek “peri” for near, “apo” for far, and “helios” for Sun. Thus we have perihelion for the closest location and aphelion for the furthest location. (These should not be confused with perigee and apogee which are the corresponding terms for moons and satellites orbiting planets or satellites orbiting moons.)
Today, our planet will be at aphelion for 2017. (The day and time vary from year to year.) I checked with NASA to find out the exact time and distance from the Sun this will happen.
Precise aphelion will occur today at 20:11 UTC/GMT or 3:11 pm CDT USA. The Earth will be 152,092,505 kilometres or 94,505,901 miles away from the Sun’s surface.
There’s something else that’s interesting and affects our seasons. After aphelion, the Earth will begin moving faster along the orbit due to gravitational pull towards the Sun. Likewise, the Earth begins moving slower along the orbit due to resistance of the Sun’s gravitational pull after perihelion, when we move further away. Orbital velocity works sort of like the gravity oriented speed of a roller coaster. This means that
Summer in the Northern Hemisphere is two or three days longer than Summer in the Southern Hemisphere.
It’s important to remember that aphelion happens at a different date than solstice. Summer solstice in the North and Winter solstice in the South already happened on June 21st.
The Blue Jay of Happiness likes the humor of astrophysicist Neil deGrasse Tyson. “Pluto’s orbit is so elongated that it crosses the orbit of another planet… you’ve got no business doing that if you want to call yourself a planet. Come on, now! There’s something especially transgressive about that.”