When is the best time to observe 21 Sagittarii?

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

Where is 21 Sagittarii located in the sky?

21 Sagittarii lies in the constellation Sagittarius at right ascension 18h 25m and declination -20° 33′.

What telescope do I need to split 21 Sagittarii?

Splitting 21 Sagittarii requires an aperture of roughly 150 mm or larger — the components are separated by about 1.7″ at magnitudes near 4.8. Larger apertures and steady seeing help resolve the pair cleanly.

21 Sagittarii — Double Star in Sagittarius

Observable Double Star Fair (31/100)

Sep: 1.7", Companion: mag 7.4

Magnitude 4.8^m DoubleStar Sagittarius (Sgr) Visible

Star Map + List + Plan

Observe

1Physical Properties

Magnitude 4.81

Spectral Type K2II bright giant

Star Color Red (B-V 1.31)

Distance 251 ly

2Position & Identifiers

RA 18h 25m 21.0s

Dec -20° 32' 30.0"

Constellation Sagittarius (Sgr)

HR 6896

HIP 90289

HD 169421

Flamsteed 21 Sgr

Double Cat 11325

SIMBAD

3How easy to split?

Primary 4.8 mag Companion 7.4 mag Separation 1.7″

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

Easy Medium Hard Very hard Impossible

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

4Visibility

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Best season May – Jul (peak: Jun)

5Multiple Star System

Separation 1.7″

Companion Mag 7.4

Companion Sp A1V

Position Angle 280°

Star Colors A: Red B: White

Discoverer JC 6

Notes

Companion is A5:.

Separation over time

Measured 1846 → 2008 (162 y)

Separation drift 1.8" → 1.7" (-0.10")

Rate -0.0006" / y

PA drift 297° → 280° (-17°, -0.105°/y)

Essentially fixed on human timescales — the same view your grandchildren will see.

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.