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The Water Jar

The Urn

A small asterism in the northern parts of Aquarius, lying near that constellation's border with Pegasus. It consists of just four stars, and represents the water jar or urn of Aquarius the Water Carrier, held aloft and upturned so that its water flows out. The 'water' in this case is represented by a stream of relatively faint stars running southward through Aquarius and forming an eastward-bending arc through the constellation. The Water Jar's flow of stars in fact extends beyond Aquarius' southern boundary, and ends with the bright star Fomalhaut in Piscis Austrinus, the Southern Fish.


The shape of the Water Jar is defined by a triangle of three stars: Sadachbia, Seat and Eta Aquarii. The formation is completed by a fourth star, Zeta Aquarii, which lies in the middle of the triangle formed by the other three.

Map of the Water Jar in Aquarius
Map of the Water Jar in Aquarius

None of these four stars is particularly bright, but nonetheless the triangular shape stands out in a rather sparse region of the sky. The brightest member of the formation is the central star, Zeta Aquarii, though this is actually a binary system composed of two bright yellow stars designated Zeta2 (the primary) and Zeta1, with a combined visual magnitude of +3.7. In individual terms, the brighest of the four stars is actually Eta Aquarii to the east, though this still has a relatively faint apparent magnitude of +4.0.

Imagery provided by Aladin sky atlas

The stars that make up the Water Jar lie close together as seen from Earth because they lie on a similar line of sight, but are in fact widely separated from one another in space. The closest of the four is the central Zeta Aquarii binary system, which is about 92 light years from the Sun, while the most distant is the northern star of the formation, Seat or Pi Aquarii. As seen from Earth, Seat is the faintest of the four stars, but in fact it is by far the most luminous member of the Water Jar formation. Seat emits nearly six hundred times as much light as the Sun, but at a distance of some 930 light years, it is more than ten times farther from Earth than Zeta Aquarii, and so appears relatively faint in the sky.

The Eta Aquariids

There are several annual meteor showers with radiant points that lie within Aquarius, but the most prominent of these is the shower known as the Eta Aquariids. From late April to late May each year, the Earth passes through a cloud of meteoric material left in the wake of Halley's Comet as it passed through the inner Solar System. The meteors of this shower appear to originate from a particular point in the sky, and this radiant point lies close to Eta Aquarii, the eastern star of the Water Jar triangle. Thus the shower is known as the Eta Aquariids, to distinguish it from the various other Aquariid showers with radiants elsewhere in Aquarius.


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