Astronomer Mark Thompson investigates why the "ice giant" Uranus is at a very special point in its orbit.
September 26th started as a pretty normal day for me; copious amounts of coffee, writing and all the usual morning stuff. Even the weather was the typical dank-grey and drizzle I've come to expect of the onset of British autumn.
But Monday wasn't just any normal day, as yesterday was the day that the mighty planet Uranus was at opposition. This means the "ice giant" is now lying opposite the sun in the sky (from Earth's perspective) giving astronomers the best chance this year to observe it.
"Hang on," I hear you all cry, "...you mean there are good times and bad times to observe the planets?" Well as it turns out, yes, in fact there are some times when they aren't even visible.
Confused? Well, let me explain more about the celestial dance of the planets.
Before looking at all the different terms in my Solar System Jargon Buster below, it's worth remembering that the orbits of the planets aren't circular, they are actually ellipses. As they travel around the sun, they will be moving faster at closest approach (perihelion) and slower when further away (aphelion), in accordance with Johannes Kepler's third law of planetary motion. In addition to this speeding up and slowing down, the planets all move at different average speeds with the closest, Mercury, moving much faster than the more distant Neptune.
You now get the picture of how they move and it's because of the differing speeds, not to mention the vast distances involved, that means their position relative to Earth and the sun changes.
Now fear not, my Solar System Jargon Buster will help you differentiate your conjunctions from your oppositions and your eastern elongations from your western ones!
Opposition: As the planets (Earth included) move around the sun, the sun and planet will appear at changing positions in the sky. When the planet lies in the opposite direction to the sun, it is said to be at opposition. At opposition, when the sun sets, the planet is just rising. It's at this point where the Earth is in between the two and the distance between the two objects is the shortest that year. It's worth noting that due to the elliptical nature of the orbits, some oppositions are closer than others. Also, it's only possible to have the outer planets (relative to Earth) at opposition; Mars through to Neptune. Mercury and Venus, this one isn't for you.
Conjunction: A conjunction exists when astronomical objects lie close to one another in the sky when viewed from Earth. "Inferior conjunctions" occur when the planet, sun and Earth line up, with the planet between us and the sun. "Superior conjunctions" are opposite to opposition! The planet lies on the other side of the sun from us here on Earth. Superior conjunctions are the worst time to observe a planet, whereas inferior conjunctions can offer unique opportunities such as the transit of Venus across the sun's disk in June 2012 which occurs whilst Venus is at inferior conjunction.
Elongations: This term is just for Mercury and Venus, the outer planets lose out. As they move around the sun, neither of the two inner planets are ever far from it in the sky when viewed from Earth. From our viewpoint, they seem to pop out from behind the sun after superior conjunction, move away from the sun, pause (at greatest elongation) and then head back toward it again. They then drift into inferior conjunction.
When the planet reaches its greatest elongation (distance in the sky) from the sun in the morning sky, it's at "greatest western elongation" and when at greatest distance in evening sky it's at "greatest eastern elongation." This is the best time to observe the innermost planets.