When is the best time to observe Algedi?

Algedi is best observed from June through August, with peak visibility during July when it transits around midnight in the Northern Hemisphere.

Where is Algedi located in the sky?

Algedi lies in the constellation Capricornus at right ascension 20h 18m and declination -12° 33′.

What telescope do I need to split Algedi?

Splitting Algedi requires an aperture of roughly 60 mm or larger — the components are separated by about 153.4″ at magnitudes near 3.6. The companion is about 6.9 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Algedi — Double Star in Capricornus

HIP 100064; Alpha2 Capricorni; 6 Capricorni

Magnitude 3.6^m DoubleStar Capricornus (Cap) Visible

Star Map + List + Plan

About Algedi

Description

Algedi, Alpha-2 Capricorni, is a G-K type giant of spectral class G8.5 III-IV about 106 light-years away. It shines at magnitude 3.57. Algedi pairs with Alpha-1 Capricorni in a naked-eye optical double visible without instruments — the two stars are actually 350 light-years apart (Alpha-1 is much farther) but look identical in separation on the sky. Algedi itself is a close triple-star system.

Observing Tips

Algedi and Alpha-1 Cap form one of the most accessible naked-eye doubles in the zodiac. The 6.6-arcminute separation is resolvable by anyone with good eyesight under dark skies; binoculars easily show both. The two stars appear similarly bright (magnitudes 3.57 and 4.24) but with subtly different colors. Best observed July through November.

History

The name Algedi comes from the Arabic "al-jady," meaning "the kid" (young goat) — a reference to the goat-figure of Capricornus. The IAU adopted the name in 2016.

Fun Facts

The Alpha-1 / Alpha-2 Cap pair is one of the best demonstrations of line-of-sight double stars for beginners: the two stars appear to be one system to the naked eye, but are at very different distances. Careful parallax measurements (Gaia) have confirmed the two are not gravitationally bound.

Observe

1Physical Properties

Magnitude 3.57

Spectral Type G8.5III-IV giant

Star Color Orange (B-V 0.94)

Distance 106 ly

2Position & Identifiers

RA 20h 18m 03.3s

Dec -12° 32' 41.0"

Constellation Capricornus (Cap)

HR 7754

HIP 100064

HD 192947

SAO 163427

Bayer Alpha2

Flamsteed 6 Cap

Double Cat 13645

SIMBAD Wikipedia

3How easy to split?

Primary 3.6 mag Companion 10.5 mag Separation 153.4″

Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Medium	Hard+	Hard
150 mm Newton 150mm Newt.	Easy	Medium+	Medium+
Celestron C8 (203 mm SCT) C8 203mm	Easy	Easy	Medium+

Easy Medium Hard Very hard Impossible

Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs

4Visibility

Set a location in User Settings to see visibility data.

Best season Jun – Aug (peak: Jul)

5Multiple Star System 9 components D,E: optical

Components 9

Component IDs *

Separation 153.4″

Companion Mag 10.5

Position Angle 159°

Star Colors A: Orange B: Orange

Discoverer AGC 12

Notes

IDS notes Alpha1 Cap = HR 7747 (ADS 13632, four components) as one of five components of Alpha2 Cap = HR | 7754 = ADS 13645. Hence, we have recorded n = 9 for both. ADS 13645AB optical; BC 11.2, 11.5v, sep. 1.2" binary.

Separation over time

Measured 1838 → 2020 (182 y)

Separation drift 198.0" → 153.4" (-44.60")

Rate -0.2451" / y

PA drift 156° → 159° (+3°, +0.016°/y)

Apparent motion is significant on a human timescale — worth revisiting in a decade.

Measured from the WDS observational archive. No orbital solution has been derived — most likely the period is too long to fit an orbit to the available measurement arc.