When is the best time to observe Tau Bootis?

Tau Bootis is best observed from March through May, with peak visibility during April when it transits around midnight in the Northern Hemisphere.

Where is Tau Bootis located in the sky?

Tau Bootis lies in the constellation Boötes at right ascension 13h 47m and declination +17° 27′.

What telescope do I need to split Tau Bootis?

Splitting Tau Bootis requires an aperture of roughly 950 mm or larger — the components are separated by about 1.5″ at magnitudes near 4.5. The companion is about 6.6 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Tau Bootis — Double Star in Boötes

Magnitude 4.4–4.6^m DoubleStar Boötes (Boo) Visible 1 Exoplanet

Star Map + List + Plan

Observe

1Physical Properties

Magnitude 4.50

Variable Type Delta Scuti (Pulsating)

Spectral Type F6IV subgiant

Star Color Yellow (B-V 0.48)

Distance 55 ly

2Position & Identifiers

RA 13h 47m 15.7s

Dec +17° 27' 24.0"

Constellation Boötes (Boo)

HR 5185

HIP 67275

HD 120136

SAO 100706

Bayer Tau

Flamsteed 4 Boo

Double Cat 9025

SIMBAD

3How easy to split?

Primary 4.5 mag Companion 11.1 mag Separation 1.5″

Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	V. hard	V. hard	V. hard
150 mm Newton 150mm Newt.	V. hard	V. hard	V. hard
Celestron C8 (203 mm SCT) C8 203mm	V. hard	V. hard	V. hard

Easy Medium Hard Very hard Impossible

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

To reach "Medium" at Bortle 3, you'd need at least a 950 mm reflector.

4Visibility

Set a location in User Settings to see visibility data.

Best season Mar – May (peak: Apr)

5Multiple Star System

Separation 1.5″

Companion Mag 11.1

Companion Sp M2V

Position Angle 76°

Star Colors A: Yellow B: Red

Discoverer STT 270

Notes

Binary, period several centuries. Companion M2.

Separation over time

Measured 1825 → 2017 (192 y)

Separation drift 22.5" → 1.5" (-21.00")

Rate -0.1094" / y

PA drift 345° → 76° (+91°, +0.474°/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.