William Herschel discovered NGC 2403 = H V-44 on 1 Nov 1788 (sweep 879) and recorded "cB; R; vgbM; BN; 6 or 7' dia; resembling a star with a misty atmosphere." His summary description, also based on a later observation, states "with a faint branch extending a great way to the np side; not less than 1/2 degree; and to the n or nf the nebulosity diffused over a space, I am pretty sure, not less than a whole degree." Obviously, his size estimate is well off, but his description of a "faint branch extending a great way to the np side" applies to the spiral side on the west side and the comment "to the n or nf the nebulosity is diffused over a space" probably applies to the northern arm that extends to the east (and contains NGC 2404).

Based on photographs taken with the 60-inch reflector at Mt Wilson, Francis Pease (1917) remarked "This fine right-handed spiral nebula resembles M33...Faint knots and arms extend as far as 10' from the center. It does not contains the wealth of detail of M33, but has the same sharp stellar images, the nebulous stars [HII regions], the bunching of these knots and the dark streaks.."

200/250mm - 8" (1/1/84): bright, large, bright core, faint star superimposed, mottled?

300/350mm - 13.1" (1/11/86): spiral arm definite on the west side of the galaxy with a dark gap between this arm and the main body (core). This feature is very faint but definite with averted.

13.1" (12/22/84): spiral arm highly suspected attached at the west side winding along the north side to a faint knot = NGC 2404.

13.1" (1/28/84): very bright, large bright core. NGC 2404 is clearly visible as a faint, very small nebulous knot along the east side of the galaxy. Spiral structure (arms) is just suspected. On 3/24/84 the knot was difficult to view at 144x, but on 1/11/86 was fairly easy.

400/500mm - 17.5" (2/28/87): very bright, very large, bright core, elongated 5:2 NW-SE, 15'x6'. Impressive galaxy with spiral structure clearly visible. Two spiral arms are attached at opposite ends of the central region and both wind almost 180°. The tip of the northern arm ends at the emission nebula NGC 2404. Several stars are superimposed including two mag 11 stars.

17.5" (2/22/87): two spiral arms are visible on attached at the opposite sides of the galaxy and winding a half of revolution. The northern arm ends at the HII knot NGC 2404. The galaxy has a mottled appearance.

900/1200mm - 48" (4/15/10): this amazing multi-arm spiral stretched across the entire 15' field at 330x, with the major axis running NW to SE. A striking prominent spiral arm is attached near the NW end and sweeps counter-clockwise 180° along the northern side of the galaxy, tapering as it extends to a point roughly 5' SE of the core. The arm is widest near the NW end, where it begins sweeping east. A second inner arm attaches near the SE end of the main body and tightly hugs the southern side of the galaxy as sweeps to the NW side. A more ill-defined branch heads west from the SE end past a mag 11 star situated 2' WSW of the core. The field was too small to trace out the outer arms. Numerous HII splotches stained the surface and I only had time to quickly sketch the most obvious knots.

The supergiant HII complex NGC 2404 located 1.7' ENE of the core and 1.5' NNW of a superimposed mag 10.5 star, appeared as a very bright, irregular, 20" knot. Hodge lists NGC 2404 as A67 in his 1985 paper "Stellar Associations in the Galaxy NGC 2403". On the NW side of the galaxy is a collinear string of 3 stars oriented NE to SW with an obvious collinear knot an additional 50" SW (SPC-44 in Sivan, Petit and Comte's 1990 "Optical HII Regions in NGC 2403"; VS 3 in Véron and Sauvayre; and A14 in Hodge). Just north of the core are two HII knots, separated by 45" and both 12" in size. The western knot is catalogued as SPC-174 = VS 24 = A36 and the eastern knot as SPC-224 = VS 38 = A45 .

On the SE side of the galaxy is a pair of HII knots separated by 40". The eastern knot is SP-346 = VS 51 = A80 and is situated 1.6' SE of the mag 10.5 star and the western knot includes SPC-318/321/322. At the SE end of the spiral arm that contains NGC 2404 is another 10" knot, which includes SPC-348/351/352 = VS 52 = A81. It can be pinpointed 1.0' NE of the mag 10.5 star. On the SE end of the galaxy a fairly faint, irregular knot (SPC-331/336/343 = VS 48/49 = A73) was noticed 1' W of a mag 14 star.

Notes by Steve Gottlieb