4 February: today’s theme is the ecliptic

Astronomers often refer to the ecliptic, but much less often say what it actually is. The answer doesn’t actually help all that much: it is the plane of the Earth’s orbit around the Sun. So what is that important? For one thing, from the Earth’s point of view, it is also the annual path of the Sun around the Earth, as seen against the background of the stars. And all the other planets orbit in more or less the same plane, so it is their path as well. And to give it a more ancient and wider meaning, it also marks the zodiac. Everyone’s heard of that!

The so-called line-up of the planets in 2025 is a good example of why the ecliptic is significant. The media are saying that the planets are all in a line, but the fact is, they always are, pretty well, because all the planets are to be found within a fairly small distance of the ecliptic, in the broader band known as the zodiac. And everyone can reel off the constellations that lie along the zodiac — Taurus, Gemini, Cancer and all the rest of them.

The constellations of the zodiac are the ones through which the Sun, Moon and planets all move.

All this was a result of the way the planets formed from a disc of material surrounding the Sun some 4.6 billion years ago. They still orbit in the same plane, though there are slight differences between the orbits. As a result, each planet usually appears slightly above or below the ecliptic, but they all move around the sky somewhere along it.

Earth’s axis is inclined at a considerable angle to the ecliptic, which is why we get seasons. When the ecliptic is high in the daytime sky the Sun is high, so we get summer, and the opposite in winter. But this also means that at night, in winter the Moon and planets are high up. Hence the reason this National Astronomy Week is being held in winter! Summer nights are a lot warmer, but the planets are much lower down in the sky and we see them through a great chunk of the atmosphere, which makes the images unsteady.

The whole sky in National Astronomy Week at 8.30 pm. The green band marks the line of the ecliptic.

At this time of year the angle of the ecliptic to the horizon is steep, so the planets are always much better displayed than in those milder autumn nights. Look at the example below showing how steep the ecliptic (the green line) is to the horizon compared with September’s new Moon. The Moon is also above the ecliptic this March and below it this September.

The new Moon is easier to see in spring than in autumn

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