Meteor showers happen because Earth passes through a wide sparse stream of particles, shed years or centuries ago from a body, usually a comet, and continuing roughly in its orbit.
The Eta Aquarid meteors of May and the Orionids of October are sister showers: the parent body of both is the most famous of comets, 1P Halley (whose designation means that it was the first comet to be recognized as periodic). The Halley orbit crosses inward “over” (north of) the October part of Earth’s orbit, and outward “under” (south of) the May part.
This space diagram shows the path of Comet Halley during the most recent of its 76-years-apart visits, in late 1985 and early 1986. The stalks from the comet’s path to the ecliptic plane are at intervals of one month. The blue arrows are sightlines from Earth to the comet, which at this visit kept its distance from us, passing behind the Sun from the evening to the morning sky.
The orbit of the comet, and therefore its meteors, is retrograde, going around the Sun in the direction opposite to that of the planets. So the meteors collide with Earth head-on. That is why they appear swift-moving, and why we see them mostly in the after-midnight hours, when we are on the front of traveling Earth.
Because the Eta Aquarid radiant is more southward in the sky than the Orionid one, and because dark night-time is shorter for Earth’s northern hemisphere in May than in October, the Eta Aquarids tend to be better for observers in countries such as Australia.
In this view from ecliptic north, a flat arrow represents Earth’s advance in 3 minutes, and an arrow on its equator represents its rotation in 3 hours. The dotted line represents only meteors from overhead; the stream is far wider than Earth. At this time, America is about to come around into view of the radiant.
How many Eta Aquarids might you see? Among the annual showers, this one ranks fairly high, with a zenithal hourly rate (ZHR) estimated as 50. But this is the number that one observer might count in one hour at the peak time if the sky is clear and dark and the radiant is overhead – which it is not