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The Path of the Sun
The invisible course traced by the Sun as it moves across the sky is one of the most important lines in the celestial sphere
The Sun never deviates from the path that it traces across the sky. We can't see this line of course but we call it the 'ecliptic' and it's one of the most important markers in the sky. Why? Because the ecliptic also represents the orbital plane of our planet.
All of the planets in the Solar System occupy orbital planes similar to our own. This is because when the Solar System formed, billions of years ago, gravity pulled the dust and gas surrounding our nascent star into a kind of flat disc. The planets we know today all formed within this disc, so they all occupy planes similar to the ecliptic — described as 'coplanar'. Simply put, when the planets are visible, they will always be near the ecliptic.
It's this coplanar nature of the Sun and planets that allows many of the events that captivate astronomers to happen so often. When our Moon and the Sun line up, we see an 'eclipse'. When a planet appears to be in the same part of the sky as another, or our own Moon, we call it a 'conjunction'. Even 'rare' events like a transit of Venus — when Venus passes between the Sun and a superior planet and appears as a small black disk moving across the face of the Sun — are quite frequent in cosmological terms.
The two points at which the ecliptic crosses the celestial equator mark the moments when the hours of day and night are roughly the same. These are called 'equinoxes', from the Latin for 'equal night'.
In the northern hemisphere, the equinox in mid-March signals spring, while the one in mid-September marks the beginning of autumn. At these two points in its orbit, Earth has no tilt relative to the Sun. From the March equinox, the days lengthen until mid-June, when Earth reaches the point in its orbit where it is at its greatest tilt relative to the Sun — a solstice. This is the first day of summer and the longest day of the year. At this point, the ecliptic and the celestial equator are at their furthest apart.
The second solstice six months later, in mid-December, when the tilt of the poles is reversed in relation to the Sun, marks the start of winter and the shortest day of the year in the northern hemisphere.
The Planets in Opposition
When the Sun, Earth and another planet form a line with Earth in the middle they are said be in 'opposition'. From our perspective on Earth, this means that the planet is in the opposite position in the sky to the Sun. This is another result of the Solar System being coplanar. This also means that only the superior planets — those with orbits further out from the Sun than Earth's — can be in opposition.
A planet at opposition is usually at its closest to Earth and therefore appears larger than at any other time. Due to its position relative to the Sun, a planet at opposition can also appear brighter than usual, making this the best time to observe the planet on a clear night.
Tracking the Ecliptic
The Sun always sits on the ecliptic, so it's easy to work out where the line is on any clear day. Looking at the whole year, we know that the Sun — and hence the ecliptic — is higher in the sky through the day in the summer months and lower during the winter. But what about at night? If you can work out the path of the ecliptic across the night sky, you can work out where you might be able to spot a planet.
In the morning, the ecliptic sits low down, but in the evening it stretches high across the sky from east to west. This makes the dusk skies the best time to see Mercury and Venus, as they never stray far from the Sun.
By dusk, the ecliptic sits at a low angle to the horizon, so any planets are hidden in atmospheric murk. The ecliptic's orientation swings from northwest-southeast in the evening to northeast-southwest in the morning.
In a reflection of the northern hemisphere spring, the ecliptic's evening path is now low down, but in the morning it stretches high across the sky from east to west making dawn the best time to spot Mercury and Venus.
In winter, the ecliptic path is quite high when it's dark and moves higher until it reaches maximum elevation at midnight. This is a great time to observe planets, as you're able to look at them through less atmosphere.
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