When is the best time to observe Arcturus?

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

Where is Arcturus located in the sky?

Arcturus lies in the constellation Boötes at right ascension 14h 16m and declination +19° 11′.

What telescope do I need to split Arcturus?

Splitting Arcturus requires an aperture of roughly 200 mm or larger — the components are separated by about 0.3″ at magnitudes near -0.0. The companion is about 3.5 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Arcturus — Double Star in Boötes

HIP 69673; Alpha Bootis; 16 Bootis

Magnitude -0.0^m DoubleStar Boötes (Boo) Visible

Star Map + List + Plan

About Arcturus

Description

Arcturus is the brightest star in the northern celestial hemisphere and the fourth brightest overall at magnitude -0.04. It is an orange giant of spectral type K1.5III, located about 37 light-years from Earth with a luminosity of roughly 170 times solar. Arcturus is a Population II star from the thick disk of the Milky Way, older and more metal-poor than the Sun.

Observing Tips

Follow the arc of the Big Dipper's handle to 'arc to Arcturus' — one of the most famous direction-finding tricks in astronomy. It blazes with a warm orange color at the base of the kite-shaped constellation Bootes. Visible from both hemispheres, it dominates the spring and summer evening sky. Best observed from April through August.

History

The name Arcturus means 'guardian of the bear' in Greek, referring to its position behind Ursa Major. Its light was used to open the 1933 Chicago World's Fair — the light had left Arcturus around the time of the previous 1893 fair. Arcturus was one of the first stars to have its proper motion measured, by Edmond Halley in 1718.

Fun Facts

Arcturus is hurtling through the Milky Way at 122 km/s on a highly inclined orbit, and belongs to the Arcturus stream — a group of stars that may have been captured from a satellite galaxy that merged with the Milky Way billions of years ago. In half a million years, Arcturus will have moved far enough to be invisible to the naked eye.