Celestial Events
Explainers for the events that make people look up.
Overview
The recurring configurations that get announced as sky events — conjunctions, oppositions, occultations, equinoxes — each have a precise geometric meaning. Knowing what the terms actually denote makes it possible to judge for yourself whether an announced event is worth going outside for.
- A conjunction is a line-of-sight alignment, not a physical proximity.
- Opposition is the best time to observe an outer planet; conjunction with the Sun is the worst.
- Occultations give some of the sharpest size measurements available for small bodies.
- Equinoxes and solstices are instants, not days.
Conjunctions and appulses
A conjunction occurs when two bodies share the same celestial longitude — they appear close together on the sky. This is purely a line-of-sight effect: two planets in conjunction may be hundreds of millions of kilometres apart, and are simply in the same direction from Earth.
Close conjunctions are genuinely striking, particularly when they involve bright planets or a thin crescent Moon. Terminology varies: strictly, an appulse is the moment of minimum apparent separation, which is often slightly offset from conjunction in longitude. Popular coverage rarely makes the distinction, and it rarely matters visually.
Opposition and solar conjunction
For an outer planet, opposition means Earth passes directly between it and the Sun. The planet then rises at sunset, is highest around midnight, and is closest to Earth for that synodic cycle — the optimal observing configuration.
The opposite case, solar conjunction, puts the planet behind the Sun from our viewpoint, making it unobservable for weeks. For inner planets the terminology differs: Venus and Mercury have inferior conjunction, passing between Earth and the Sun, and superior conjunction, passing behind it. Greatest elongation — maximum angular separation from the Sun — is when they are best placed.
Occultations and transits
- An occultation is one body passing in front of another and hiding it: the Moon regularly occults stars and occasionally planets.
- Asteroid occultations of stars are scientifically valuable. Timing the disappearance and reappearance from multiple sites across the shadow track reconstructs the asteroid's profile, giving size and shape measurements that rival spacecraft data — and much of this work is done by amateur observers.
- A transit is a small body crossing a much larger disc. Mercury transits the Sun about a dozen times a century; Venus transits in pairs separated by more than a century, most recently in 2004 and 2012.
- The same geometry, applied to other stars, is the transit method that has found most known exoplanets.
Equinoxes and solstices
These are specific instants, not days. The equinox is the moment the Sun crosses the celestial equator; the solstice is the moment it reaches its greatest declination north or south. Both are calculable to the minute, and because they are instants they fall on different calendar dates in different time zones.
Two related misconceptions are worth correcting. Day and night are not exactly equal at the equinox — atmospheric refraction and the fact that sunrise is defined by the Sun's upper edge both extend daylight slightly, so the date of exactly equal day and night falls a few days off the equinox. And the solstices are not the hottest or coldest days: thermal lag in oceans and land means peak temperatures follow weeks later.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- What is a conjunction?
- Two celestial bodies appearing close together in the sky because they share nearly the same celestial longitude. It is a line-of-sight alignment only — the objects may be hundreds of millions of kilometres apart. Close conjunctions of bright planets, or of a planet with a crescent Moon, are among the more attractive naked-eye sights.
- Why do astronomers care about occultations?
- Because they give exceptionally precise measurements. Timing when a small body blocks a background star, from several sites spread across the shadow track, reconstructs the body's silhouette and yields size and shape data comparable to what a spacecraft flyby provides. Much of this work is carried out by coordinated amateur observers.
- Is day exactly equal to night on the equinox?
- Not quite. Sunrise and sunset are defined by the Sun's upper edge rather than its centre, and atmospheric refraction lifts the Sun's apparent position when it is near the horizon. Both effects lengthen daylight slightly, so the date of genuinely equal day and night falls a few days before or after the equinox depending on latitude.
- Why isn't the summer solstice the hottest day?
- Because of thermal lag. The solstice is when solar input peaks, but oceans and land continue absorbing more heat than they release for weeks afterwards, so peak temperatures typically arrive well after the solstice. The same lag delays the coldest part of winter past the December solstice.