When is the best time to observe V638 Cassiopeiae?

V638 Cassiopeiae is best observed from August through October, with peak visibility during September when it transits around midnight in the Northern Hemisphere.

Where is V638 Cassiopeiae located in the sky?

V638 Cassiopeiae lies in the constellation Cassiopeia at right ascension 23h 03m and declination +55° 14′.

What telescope do I need to split V638 Cassiopeiae?

Splitting V638 Cassiopeiae requires an aperture of roughly 60 mm or larger — the components are separated by about 18.6″ at magnitudes near 6.5. The companion is about 3.7 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

V638 Cassiopeiae — Double Star in Cassiopeia

Magnitude 6.5^m DoubleStar Cassiopeia (Cas) Visible

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Observe

1Physical Properties

Magnitude 6.50

Variable Type Alpha2 CVn (Magnetic Rotating)

Spectral Type B9IIIHe wk giant

Star Color Blue-white (B-V -0.08)

2Position & Identifiers

RA 23h 02m 43.8s

Dec +55° 14' 11.0"

Constellation Cassiopeia (Cas)

HR 8770

HIP 113797

HD 217833

SAO 35092

Variable ID V638 Cas

Double Cat 16474

SIMBAD

3How easy to split?

Primary 6.5 mag Companion 10.2 mag Separation 18.6″

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

Easy Medium Hard Very hard Impossible

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

4Visibility

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Best season Aug – Oct (peak: Sep)

5Multiple Star System Quadruple B: optical

Components 4 (quadruple)

Component IDs AB

Separation 18.6″

Companion Mag 10.2

Position Angle 49°

Star Colors A: Blue-white B: Yellow

Discoverer STT 485

Separation over time

Measured 1843 → 2021 (178 y)

Separation drift 21.6" → 18.6" (-3.00")

Rate -0.0169" / y

PA drift 54° → 49° (-5°, -0.028°/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.