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When LBN 864 is up, how long it stays usable and when it is in season all depend on where you observe from. Set your site and this page adds tonight's answer and its year.
The science
In one sentence
Interstellar hydrogen ionised by the ultraviolet light of hot, young stars embedded in it.
Class explainer for H II region (SIMBAD classification).
LBN 864 is an emission nebula in Orion. SIMBAD gives its distance as 1.74 kpc (−0.08 kpc / +0.07 kpc) — ≈ 5,680 ly, measured by parallax from 1 measurement. The light reaching us from it now left it about 5,680 years ago, around the end of the last ice age. SIMBAD measures it at 21′ × 21′ on the sky — 69% of the full Moon's width. At that distance, that angle means it is roughly 35 light-years across. Framing uses its catalogue size instead: 30′.
Sources: AstroBrain catalogue; Lynds B.T. 1965, ApJS 12, 163, via CDS/VizieR VII/9; CDS/SIMBAD astronomical database, Strasbourg (SIMBAD record LBN 194.20-01.99 · distance from 2025A&A...696A..67S). Contains information from SIMBAD, which is made available here under the Open Database License (ODbL). Download the SIMBAD-derived data.
35 ly
Sky image
Observatory photos and open-licence sky surveys.
Credits on each image.
Sky maths: apparent positions with refraction, one-minute steps; usable = full astronomical dark, at least 30° up and clear of your skyline. Survey images: CDS, Strasbourg (DSS2 and other surveys). Science: SIMBAD. The hours, heights and Moon on this page use no weather forecast; the rank is this target’s place in the Targets list, whose score also counts the cloud forecast when there is one.