The stars may seem eternal, but some of the most amazing sky sights are fleeting phenomena.
What happens 13 or 14 times a century, but only in May or November? Alert Sky & Telescope readers will know the answer: Mercury’s solar transit, when our system's innermost planet appears to cross the face of the Sun. The next transit will take place over the course of five and a half hours on November 11, 2019.
A transit of Mercury is one of the great set-pieces in the theater of astronomy. Its allure stems, quite literally, from its transience: it exists only in the moment. As the venerable British actress Maggie Smith once said, "I like the ephemeral thing about theater. Every performance is like a ghost. It's there and then it's gone."
What are the other great ephemeral moments in the sky? Here is a list of the top ten fleeting phenomena in astronomy. Needless to say, there is a large dose of subjectivity in all of this. We'd love to hear your thoughts in the comments below!
10. An Overhead Pass of the International Space Station
The third brightest object in the sky used to be Venus. It's now the International Space Station, a football-pitch-sized collection of laboratories, living modules, solar panels, heat radiators, trusses, robotic arms and docked spacecraft.
> More info: Spot the Station.
9. An Occultation
There is nothing in astronomy that can be compared to the flick of a light switch, except for the sudden, breathtaking disappearance of a star behind the dark edge of the Moon. A real now-you-see-it-now-you-don't moment of magic.
> More info: An Occultation Primer.
8. A Comet
Comets spend most of their life in the outer reaches of the solar system. When they briefly grace our skies, they can cause quite a stir: think of the trepidation that greeted the return of Halley's Comet in 1910.
> More info: Comets.
Speaking of comets, meteors are what we get when the Earth passes through the trail of debris that a comet leaves behind. The popular name for this phenomenon does a better job of capturing its charm: shooting stars.
> More info: Meteors.
6. A Supernova
It's a mind-boggling thought that for a few weeks, a single star might outshine its entire galaxy. Observing supernovae requires telescope skills, though; in the last thousand years, there have been only three naked-eye supernova explosions: SN 1572, SN 1604 and SN 1987A.
> More info: Latest Supernovae.
5. A Planetary Transit
Telescope skills — including the proper use of telescope solar filters — are likewise required for observing a Mercury transit. The transit of Venus didn’t require any magnification because the planet is larger against the face of the Sun . . . but the next Venus transit won't occur until 2117.
> More info: Planet Transits Worldwide.
4. A Lunar Eclipse
There is something quite spine-tingling about watching the Moon being eaten away by the Earth's shadow and turning a striking shade of red. A lunar eclipse is visible from anywhere on the night-time side of the Earth.
> More info: Lunar Eclipses Worldwide.
3. An Aurora
When the Sun’s magnetic field touches Earth’s, energetic particles shoot along magnetic field lines, mostly into the sky above Earth’s polar regions. And while the popular term for this phenomenon is Northern Lights, Earth has two poles — Southern observers can catch the lights, too!
> More info: An Aurora Watcher's Guide.
2. A Solar Eclipse
Ah, how can one describe the feeling of seeing a total solar eclipse? S&T's Monica Young gave an exquisite answer when asked to describe the mood of her group in Wyoming immediately after totality in 2017: "Just celebratory."
> More info: Solar Eclipses Worldwide.
1. Sunrise and Sunset
"If we could imagine a day prolonged for a lifetime, or nearly so, and that sunrise and sunset were rare events which happened but a few times to each of us, we should certainly be entranced by the beauty of the morning and evening tints." That — from John Lubbock, a 19th-century naturalist, politician and friend of Charles Darwin — is an understatement. Quite simply, sunrise and sunset are an order of magnitude different to any other astronomical event.
> More info: Tiptoe Into The Twilight Zone.