When is the best time to observe 39 Cancri?

39 Cancri is best observed from December through February, with peak visibility during January when it transits around midnight in the Northern Hemisphere.

Where is 39 Cancri located in the sky?

39 Cancri lies in the constellation Cancer at right ascension 08h 40m and declination +20° 00′.

What telescope do I need to split 39 Cancri?

Splitting 39 Cancri requires an aperture of roughly 60 mm or larger — the components are separated by about 151.8″ at magnitudes near 6.4. Larger apertures and steady seeing help resolve the pair cleanly.

39 Cancri — Double Star in Cancer

Magnitude 6.4^m DoubleStar Cancer (Cnc) Visible

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Observe

1Physical Properties

Magnitude 6.39

Spectral Type G8III giant

Star Color Orange (B-V 0.98)

2Position & Identifiers

RA 08h 40m 06.4s

Dec +20° 00' 28.0"

Constellation Cancer (Cnc)

HR 3427

HIP 42516

HD 73665

SAO 80333

Flamsteed 39 Cnc

SIMBAD

3How easy to split?

Primary 6.4 mag Companion 6.6 mag Separation 151.8″

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Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Easy	Easy	Easy
150 mm Newton 150mm Newt.	Easy	Easy	Easy
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 Dec – Feb (peak: Jan)

5Multiple Star System Quintuple R: optical

Components 5 (quintuple)

Component IDs AB

Separation 151.8″

Companion Mag 6.6

Position Angle 150°

Star Colors A: Orange

Discoverer ENG 37

Notes

Components B, 6.60V, +0.01(B-V), +0.03(U-B), A0III at 150", vsini =<40k/s; P, 9.22V, +0.39(B-V), +0.01(U-B) at 134"; and | Q, 8.81V, +0.32(B-V), +0.09(U-B), F2V, at 135", vsini 86k/s, form a physical system with A. B is 40 Cnc; Q is +20d2161.

Separation over time

Measured 1831 → 2015 (184 y)

Separation drift 150.0" → 151.8" (+1.80")

Rate +0.0098" / y

PA drift 151° → 150° (-1°, -0.005°/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.