Hercules

Keystone of Hercules

Hercules Keystone

common observer pattern · high confidence

The lopsided four-star keystone anchors Hercules and points the way to M13 on the western edge. From 50°N it is a high summer pattern, best when the Milky Way is climbing.

Central RA: 16^h 54^m 52.6^s
Central Dec: +34° 33′ 55″
Brightest member: V 2.81
Best months from 50°N: June–September evenings
Suggested instrument: naked-eye
Approx. span: 8.8°

Finder context

This wider chart is deliberately schematic: it uses nearby bright-star context and boxes the asterism’s member-star footprint, but it does not draw official constellation boundaries or promise horizon/season precision.

Framing: Approximate member-star span: 8.8°; use at least 12.3° field for context.

Observing and imaging

Naked eye

Primary naked-eye pattern; suburban skies should show the main stars unless the description notes a low horizon or dark-sky need.

Binoculars

Binoculars are optional: use them to check colours, nearby doubles, or richer Milky Way background.

Small scope

A telescope is usually too narrow for the whole shape; use it after the pattern has guided you to a target.

Imaging

Frame as a wide-field scene in/near Hercules; a field of view around 12° keeps context without claiming exact constellation boundaries.

Tonight from your latitude

Observability from your latitude: uses this asterism’s centroid RA/Dec with a schematic local clock from longitude, astronomical twilight (Sun below −18°), and the 2026 Moon heuristic. Month stays for the seasonal readout.

Latitude 50°NLongitude -114°Date Month Sky darkness Bortle 4 · limit V 6.1

Naked-eye visibility by sky class

Approximate limiting magnitudes: Bortle 3 ≈ V 6.6, Bortle 5 ≈ V 5.6, Bortle 7 ≈ V 4.6. The shape is counted recognisable when at least 70% of defining stars clear the limit.

Bortle 3: 4/4 stars — fully visibleBortle 5: 4/4 stars — fully visibleBortle 7: 4/4 stars — fully visible

Moon impact

Reference year 2026, Jul: 30/31 nights (97%) have a sampled ≥2 h window with Keystone of Hercules above 20° and the Moon below the horizon. At transit the Moon is up on about 52% of nights; the average moonlight penalty is roughly 0.34 mag of naked-eye limiting magnitude. Good nights below are the deterministic 2026 best-month mini-calendar for 50°N; green means a ≥2 h sampled dark window with the Moon down.

01020304050607080910111213141516171819202122232425262728293031

Member stars

Name	Bayer / Flamsteed	HR	RA J2000	Dec J2000	V mag
40Zet Her	40Zet Her	HR 6212	16^h 41^m 17.2^s	+31° 36′ 11″	2.81
67Pi Her	67Pi Her	HR 6418	17^h 15^m 02.8^s	+36° 48′ 33″	3.16
44Eta Her	44Eta Her	HR 6220	16^h 42^m 53.8^s	+38° 55′ 20″	3.53
58Eps Her	58Eps Her	HR 6324	17^h 00^m 17.4^s	+30° 55′ 35″	3.92

Source and confidence

common observer pattern; high confidence. Commonly used constellation-part or seasonal guide-pattern name, with member-star positions plotted from BSC5.

Citations

Bright Star Catalogue, 5th Revised Ed.: Hoffleit, D. & Warren, W.H. Jr. (1991). Positions, V magnitudes, Bayer/Flamsteed identifiers. source
Star Tales: Ridpath, I. Constellation and asterism notes used to cross-check common sky-pattern names. source
The Hipparcos and Tycho Catalogues: ESA SP-1200 (1997). Used as the modern astrometric reference named by the atlas; the MVP pages expose BSC5 J2000 fields for the plotted bright-star members.