When is the best time to observe HIP 20138?

HIP 20138 is best observed from October through December, with peak visibility during November when it transits around midnight in the Northern Hemisphere.

Where is HIP 20138 located in the sky?

HIP 20138 lies in the constellation Eridanus at right ascension 04h 19m and declination -33° 54′.

What telescope do I need to split HIP 20138?

Splitting HIP 20138 requires an aperture of roughly 80 mm or larger — the components are separated by about 5.2″ at magnitudes near 6.4. Larger apertures and steady seeing help resolve the pair cleanly.

HIP 20138 — Double Star in Eridanus

Magnitude 6.4^m DoubleStar Eridanus (Eri) Visible

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Observe

1Physical Properties

Magnitude 6.37

Spectral Type A3V

Star Color White (B-V 0.13)

Distance 466 ly

2Position & Identifiers

RA 04h 19m 03.0s

Dec -33° 54' 18.0"

Constellation Eridanus (Eri)

HR 1359

HIP 20138

HD 27490

SAO 194923

SIMBAD

3How easy to split?

Primary 6.4 mag Companion 8.7 mag Separation 5.2″

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Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Easy	Easy	Medium+
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 Oct – Dec (peak: Nov)

5Multiple Star System

Separation 5.2″

Companion Mag 8.7

Position Angle 157°

Star Colors A: White B: Yellow

Discoverer HJ 3642

Notes

Secondary F7V, vsini 90k/s.

Separation over time

Measured 1837 → 2020 (183 y)

Separation drift 5.8" → 5.2" (-0.60")

Rate -0.0033" / y

PA drift 158° → 157° (-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.