When is the best time to observe Tau Eridani?

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

Where is Tau Eridani located in the sky?

Tau Eridani lies in the constellation Eridanus at right ascension 03h 20m and declination -21° 45′.

What telescope do I need to split Tau Eridani?

Splitting Tau Eridani requires an aperture of roughly 80 mm or larger — the components are separated by about 5.8″ at magnitudes near 3.7. The companion is about 5.8 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Tau Eridani — Double Star in Eridanus

HIP 15474; Tau4 Eridani; 16 Eridani

Magnitude 3.6–3.7^m DoubleStar Eridanus (Eri) Visible

Star Map + List + Plan

About Tau Eri

Description

Tau Eridani is a red giant of spectral class M3 III-IIIa Ca-1 about 304 light-years away. It shines at magnitude 3.69 and is a semiregular variable, with brightness variations of about 0.1 magnitudes on a 30-day period. Tau Eri is a mature asymptotic-giant-branch star approaching the end of its life.

Observing Tips

Tau Eri sits in the northern part of Eridanus. Its deep orange-red color is striking in binoculars. Long-term amateur monitoring can track the variability, though the amplitude is modest. Best observed October through February.

History

Tau Eri retains its Bayer designation in current IAU catalogs.

Fun Facts

Tau Eri's M3 spectrum shows strong molecular absorption bands from titanium oxide, calcium hydride, and other molecules — signatures of the cool outer envelope of a red giant. Its mass-loss rate is high enough that the star is shedding material into a growing circumstellar shell.

Observe

1Physical Properties

Magnitude 3.69

Range 3.57 - 3.78

Period 23.8 days

Variable Type Semi-Regular Variable (Short Period)

Spectral Type M3III-IIIa Ca-1 giant

Star Color Red (B-V 1.62)

Distance 304 ly

2Position & Identifiers

RA 03h 19m 31.0s

Dec -21° 45' 28.0"

Constellation Eridanus (Eri)

HR 1003

HIP 15474

HD 20720

SAO 168460

Bayer Tau4

Flamsteed 16 Eri

Variable ID Tau4 Eri

Double Cat 2472

SIMBAD Wikipedia

3How easy to split?

Primary 3.7 mag Companion 9.5 mag Separation 5.8″

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	Medium	Medium	Medium

Easy Medium Hard Very hard Impossible

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

4Visibility

Set a location in User Settings to see visibility data.

Best season Oct – Dec (peak: Nov)

5Light Curve

6Multiple Star System Sextuple C,D,E,F: optical

Components 6 (sextuple)

Component IDs AB

Separation 5.8″

Companion Mag 9.5

Companion Sp K0V

Position Angle 290°

Star Colors A: Red B: Orange

Discoverer JC 1

Notes

AB binary, B is K0V. C, 11v at 40" optical.

Separation over time

Measured 1857 → 2016 (159 y)

Separation drift 5.5" → 5.8" (+0.30")

Rate +0.0019" / y

PA drift 287° → 290° (+3°, +0.019°/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.