When is the best time to observe Alnilam?

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

Where is Alnilam located in the sky?

Alnilam lies in the constellation Orion at right ascension 05h 36m and declination -01° 12′.

What telescope do I need to split Alnilam?

Splitting Alnilam requires an aperture of roughly 60 mm or larger — the components are separated by about 179.5″ at magnitudes near 1.7. The companion is about 9.6 magnitudes fainter, so glare from the primary — not separation — sets the minimum aperture. Larger apertures and steady seeing help resolve the pair cleanly.

Alnilam — Double Star in Orion

HIP 26311; Epsilon Orionis; 46 Orionis

Magnitude 1.6–1.7^m DoubleStar Orion (Ori) Visible

Star Map + List + Plan

About Alnilam

Description

Alnilam is a blue supergiant of spectral type B0Ia, the middle and brightest star of Orion's Belt at magnitude 1.70. Located about 2,000 light-years from Earth, it is one of the most distant bright stars visible to the naked eye. Alnilam is extraordinarily luminous — roughly 275,000 times the luminosity of the Sun — and has about 40 times the Sun's mass. It is losing mass rapidly through powerful stellar winds.

Observing Tips

Alnilam is the middle star of Orion's famous Belt, flanked by Alnitak to the east and Mintaka to the west. The three Belt stars are one of the most recognizable patterns in the night sky. Through binoculars, the surrounding region is rich with nebulosity — the emission nebula NGC 1990 surrounds Alnilam itself. The Belt points downward to Sirius and upward to Aldebaran, making it an excellent starting point for star-hopping. Best observed from November through March.

History

The name Alnilam comes from the Arabic 'al-Nilam' meaning 'string of pearls,' referring to the line of Belt stars. Orion's Belt is one of the most universally recognized star patterns across human cultures — it has been identified with three kings, three sisters, a celestial bridge, and many other mythological figures worldwide. The ancient Egyptians aligned the three Great Pyramids of Giza to mirror the Belt stars.

Fun Facts

At roughly 275,000 solar luminosities, Alnilam is one of the most luminous stars known in the Milky Way. Its powerful stellar wind blows at over 2,000 km/s, and it is losing mass at a rate millions of times greater than the Sun's solar wind. The alignment of the three Pyramids of Giza with Orion's Belt (the Orion Correlation Theory) is one of the most popular — and debated — claims in archaeoastronomy.

Observe

1Physical Properties

Magnitude 1.70

Range 1.64 - 1.74

Variable Type ACYG

Spectral Type B0Ib supergiant

Star Color Blue (B-V -0.19)

Temperature 27526 K

Radius 32.4 R☉

Distance 1,341 ly

2Position & Identifiers

RA 05h 36m 12.8s

Dec -01° 12' 07.0"

Constellation Orion (Ori)

HR 1903

HIP 26311

HD 37128

SAO 132346

Bayer Epsilon

Flamsteed 46 Ori

Variable ID f

SIMBAD Wikipedia

3How easy to split?

Primary 1.7 mag Companion 11.3 mag Separation 179.5″

Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Hard+	Hard	V. hard+
150 mm Newton 150mm Newt.	Medium+	Medium	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 Nov – Jan (peak: Dec)

5Multiple Star System

Separation 179.5″

Companion Mag 11.3

Position Angle 58°

Star Colors A: Blue

Discoverer BUP 81

Separation over time

Measured 1835 → 2023 (188 y)

Separation drift 160.0" → 179.5" (+19.50")

Rate +0.1037" / y

PA drift 68° → 58° (-10°, -0.053°/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.