Near-Earth asteroid flybys on the timelines of your cities: every asteroid passing closer than the Moon, and the large ones - about 140 metres across and bigger - passing within 0.05 astronomical units, about 20 lunar distances. Each sits at the time of its closest approach, the same moment for the whole Earth; the bar is that moment plus or minus its time uncertainty, at least an hour wide.
The data comes from NASA/JPL's close-approach database, which computes every approach from the orbits of all known asteroids. Small asteroids are often discovered only days before they pass, so the calendar is refreshed every few hours and new flybys appear as they are found. Asteroids closer than the Moon have one colour, the large ones another.
Select a flyby for the time of closest approach in UTC, the distance in lunar distances and kilometres with its uncertainty range, the size - measured, or estimated from the asteroid's brightness - the speed relative to the Earth and a link to its orbit and data at JPL.
Every asteroid passing closer than the Moon, and every large one passing within about 20 lunar distances, is on the timeline of each of your cities at the time of its closest approach. Small asteroids are often found only days before they pass, so new ones appear as they are discovered - the calendar is refreshed every few hours.
Select a flyby for its distance in lunar distances and kilometres, measured between the centres of the Earth and the asteroid, and for the range the orbit's uncertainty allows. One lunar distance is about 384,400 km.
The card gives the measured size where there is one. Otherwise it is estimated from the asteroid's brightness, as a range, because the same brightness means a small shiny rock or a larger dark one.
Almost never with the naked eye - even a close one is a faint moving dot that needs a telescope, and the small ones pass quickly. The calendar shows when they pass, not whether they are visible.
In Astronomy. Perfect Clock — a world clock with your calendar across time zones.