When is the best time to observe Eta Sagittarii?

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

Where is Eta Sagittarii located in the sky?

Eta Sagittarii lies in the constellation Sagittarius at right ascension 18h 18m and declination -36° 46′.

What telescope do I need to split Eta Sagittarii?

Splitting Eta Sagittarii requires an aperture of roughly 210 mm or larger — the components are separated by about 3.5″ at magnitudes near 3.1. The companion is about 4.9 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Eta Sagittarii — Double Star in Sagittarius

Observable Double Star Excellent (61/100)

Sep: 3.5", Companion: mag 8.0

Magnitude 3.1^m DoubleStar Sagittarius (Sgr) Visible

Star Map + List + Plan

Observe

1Physical Properties

Magnitude 3.11

Range 3.05 - 3.12

Variable Type LB:

Spectral Type M2 III giant

Star Color Red (B-V 1.56)

Distance 146 ly

2Position & Identifiers

RA 18h 17m 37.6s

Dec -36° 45' 42.0"

Constellation Sagittarius (Sgr)

HR 6832

HIP 89642

HD 167618

SAO 209957

Bayer Eta

Variable ID Eta Sgr

SIMBAD

3How easy to split?

Primary 3.1 mag Companion 8.0 mag Separation 3.5″

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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	Hard+	Hard+	Hard+

Easy Medium Hard Very hard Impossible

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

To reach "Medium" at Bortle 3, you'd need at least a 210 mm reflector.

4Visibility

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Best season May – Jul (peak: Jun)

5Multiple Star System Quadruple D: optical

Components 4 (quadruple)

Component IDs AB

Separation 3.5″

Companion Mag 8.0

Position Angle 110°

Star Colors A: Red

Discoverer BU 760

Separation over time

Measured 1879 → 2016 (137 y)

Separation drift 2.8" → 3.5" (+0.70")

Rate +0.0051" / y

PA drift 100° → 110° (+10°, +0.073°/y)

Slow change over generations — observable in lifetime comparisons.

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.