A small object passing in front of a larger one without hiding it: Mercury across the Sun (next on 13 November 2032), one of Jupiter’s moons across the planet, or the ISS across the Sun or the Moon. For the Sun, a solar filter is essential.
ToolsISS passes
Related terms
- Occultation
One object passing in front of another and hiding it completely, most often the Moon in front of a star or planet. A star vanishes instantly at the Moon’s edge, since the Moon has no atmosphere. Timing these moments precisely is a useful amateur contribution.
- Galilean moons
Jupiter’s four large moons, Io, Europa, Ganymede and Callisto, discovered by Galileo in 1610. Small, steadily held binoculars show them as points lined up on either side of the planet. Their dance changes hour by hour: eclipses, transits across the disc, shadows.
- ISS (International Space Station)
The largest crewed satellite, orbiting about 400 km up and circling the Earth in about 90 minutes. It looks like a very bright, steady point crossing the sky in a few minutes. It is only visible around dawn or dusk, when it is sunlit while the observer is in darkness.
- Solar filter
The only safe way to look at the Sun: a certified film or glass fixed securely over the front of the instrument, blocking more than 99.99% of the light. Never use a filter that screws onto the eyepiece: it can crack from the heat. A single glance through an unfiltered instrument causes blindness in a fraction of a second.
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