Happy Perihelion!

Hi I’m Lisa the director of learning at the Space Centre.
When does the ‘new year’ begin? Most countries use the Gregorian calendar which marks January 1 as the beginning of the new year. There are a number of cultural traditions that base a calendar on the Moon and/or the Sun and celebrate the new year based on astronomical events. Examples of these include Chinese New Year which is celebrated on the second new Moon after the winter solstice or the Persian New Year, celebrated on the spring equinox. Another astronomical event that marks the beginning of a new year is perihelion, the point at which Earth is closest to the Sun. This year, perihelion occurred on January 3. 

An orbit is the path an object travels as it goes around another object. Understanding how planets orbit was an important point in astronomy. In the early 1500s, the idea that Earth has an orbit was revolutionary. Prior to the 1500s, astronomers adopted the belief that Earth was stationary and the planets and Sun orbited around it. Copernicus was the first to observe the planets revolving around the Sun (1515). But it took another 100 years for the planet’s orbits to be defined. The modern understanding of planetary motion begins with Johannes Kepler. In 1605, Kepler described the shape of an orbit as elliptical not circular with his first law of planetary motion. Because of the elliptical shape, Earth’s orbit has a perihelion (closest to the Sun) and an aphelion (farthest from the Sun). Kepler’s laws were a good start at understanding orbits as they described how planets moved but didn’t explain why. 

That understanding was developed by Isaac Newton. Building on Kepler’s laws, he developed three universal laws of motion in 1687 to explain the movement of the planets in our solar system, for the most part. One thing Newton’s theory did not explain was Mercury’s unusual orbit. Because Mercury’s ellipse isn’t ‘fixed’ but gradually rotates, its closest point to the Sun drifts. This drift also happens to our orbit, but at a much slower rate. This will be noticeable in about 3200 years when the Polaris is no longer the ‘north star’ but a star in the constellation Cepheus (Gamma Cephei). 

This new understanding of how orbits worked helped to expand our understanding of our solar system. It led to the discovery of Neptune, the first planet to be discovered using mathematics. Astronomers noticed that Uranus, discovered in 1781, was pulled slightly out of its expected elliptical orbit. Using math, astronomers predicted that the gravity from another planet was affecting Uranus and calculated the mass and location of this planet. Using this information, astronomer Johann Gottfried Galle searched for the predicted planet and found it for the first time in 1846. This was a critical point in astronomy as it marks the point where mathematics and theory rather than observation led to new discoveries.
Orbits continue to play a role in defining our solar system. Astronomers studying objects in the Kuiper Belt have noticed that some of them tend to follow orbits that cluster together. By analyzing the orbits, they have predicted the possibility of another, large planet orbiting far out in the solar system, so far that its year would be equivalent to 10,000 to 20,000 Earth years. Astronomers are searching for this planet in its predicted orbit but as of yet have not observed it.
So, as we start a new year, whether you observed it based on the Gregorian calendar or by the location of Earth relative to the Sun, take a moment to discover more about how planets orbit with these activities.

Astronomers Playlist

Orbiting Planets
Here are three short articles about orbits and planets:

Planetary motion: the history of an idea that launched a scientific revolution

Astronomers discover Neptune, the eight planet

If Planet Nine exists why has no one seen it?

Ask yourself: What do you think about the different theories about Planet Nine?


Speed and Gravity

What affects a planet’s orbit? Try these activities to find out how speed, gravity and mass affect an object’s orbit.

The kitchen bowl orbit (easy)

Create your own virtual objects to see how they orbit each other

Modify the mass of a planet to see how it affects the inner solar system

Ask yourself: How did changing aspects such as speed and mass affect orbits? What do you think might happen if the orbit of one planet becomes unstable?


The Sounds of Orbits

The TRAPPIST-1 system was discovered in 2017. Its seven planets orbit in a complex predictable pattern called orbital resonance. Discover more about this solar system and how astronomers make use of orbital resonance.

Astronomers confirm orbital details of TRAPPIST-1’s least understood planet

The TRAPPIST-1 system

Listen to the sounds of TRAPPIST-1 planet orbits

Make your own music using the TRAPPIST-1 planets as your instruments

Ask yourself: Sonification is the process of changing data into sound. Google sonification in astronomy. Can you find out why astronomers might want to listen to data?