When is the best time to observe 40 Persei?

40 Persei is best observed from October through December, with peak visibility during November when it transits around midnight in the Northern Hemisphere.

Where is 40 Persei located in the sky?

40 Persei lies in the constellation Perseus at right ascension 03h 42m and declination +33° 58′.

What telescope do I need to split 40 Persei?

Splitting 40 Persei requires an aperture of roughly 60 mm or larger — the components are separated by about 19.9″ at magnitudes near 5.0. The companion is about 5.1 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

40 Persei — Double Star in Perseus

Magnitude 5.0^m DoubleStar Perseus (Per) Visible

Star Map + List + Plan

Observe

1Physical Properties

Magnitude 4.97

Spectral Type B0.5V

Star Color Blue (B-V -0.01)

2Position & Identifiers

RA 03h 42m 22.5s

Dec +33° 57' 54.0"

Constellation Perseus (Per)

HR 1123

HIP 17313

HD 22951

SAO 56646

Flamsteed 40 Per

Double Cat 2699

SIMBAD

3How easy to split?

Primary 5.0 mag Companion 10.0 mag Separation 19.9″

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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 Oct – Dec (peak: Nov)

5Multiple Star System

Separation 19.9″

Companion Mag 10.0

Position Angle 239°

Star Colors A: Blue

Discoverer STF 431

Separation over time

Measured 1828 → 2016 (188 y)

Separation drift 19.8" → 19.9" (+0.10")

Rate +0.0005" / y

PA drift 237° → 239° (+2°, +0.011°/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.