When is the best time to observe Yed Posterior?

Yed Posterior is best observed from April through June, with peak visibility during May when it transits around midnight in the Northern Hemisphere.

Where is Yed Posterior located in the sky?

Yed Posterior lies in the constellation Ophiuchus at right ascension 16h 18m and declination -04° 42′.

What telescope do I need to see Yed Posterior?

At magnitude 3.2, Yed Posterior is visible to the naked eye from dark skies, easy in any binoculars.

Yed Posterior — Star in Ophiuchus

HIP 79882; Epsilon Ophiuchi; 2 Ophiuchi

Magnitude 3.2^m Star Ophiuchus (Oph) Visible

Star Map + List + Plan

About Yed Posterior

Description

Yed Posterior, Epsilon Ophiuchi, is a G-type giant of spectral class G9.5 IIIb lying about 106 light-years from Earth. It shines at magnitude 3.24 and has a mild iron underabundance compared to the Sun. With a radius of 11 Suns and a luminosity of 54 Suns, it is a typical evolved first-ascent red-clump giant — a star that is currently fusing helium in its core. It forms a naked-eye optical pair with Yed Prior (Delta Oph).

Observing Tips

Yed Posterior sits just east of Yed Prior, the two forming a prominent 0.5-degree-apart pair along the left shoulder of Ophiuchus. Binoculars show both stars clearly, with Yed Posterior appearing the warmer of the two. The names encode their positions: Prior (western) rises first, Posterior (eastern) rises second, from the geocentric viewpoint of pre-modern astronomy. Best observed April through September.

History

The name comes from the Arabic "al-yad," meaning "the hand," with the Latin "Posterior" added in medieval compilations to distinguish it from its neighbor Yed Prior. Epsilon Oph was formally assigned the name Yed Posterior by the IAU in 2016.

Fun Facts

Yed Posterior's position near the ecliptic means it is occasionally occulted by the Moon and, rarely, by asteroids — valuable events for timing observations. Its spectrum has been used as a template for understanding similar K-type red-clump giants throughout the galaxy. The two Yed stars (Prior and Posterior) are an optical pair: at 104 and 170 light-years respectively, they are not physically related.