{"release":{"id":"DR1","released_on":"2026-08-21","doi":null,"url":"https://spacecatalog.org/releases#dr1"},"epoch":{"requested":"now","label":"J2026.677","julian_year":2026.677,"time":"2026-09-05T10:04:39.288Z","years_from_j2000":26.677,"frame":"date","applied":"Proper motion, then precession and nutation. The catalogued J2000 position is carried along the great circle the object's own published motion puts it on, at a constant velocity, and the result is referred to the true equator and equinox of this instant. Those are separate things: the motion moves the object, and the rotation turns the grid under it by about fifty arcseconds a year, with nutation adding up to another seventeen.","excluded":"Parallax, the radial-velocity perspective term, aberration and refraction; each is below the accuracy of the catalogued positions themselves. An object with no published proper motion comes back where the catalogue has it rather than at a fabricated zero, and says so with a null pm_ra_mas_yr. A body with no fixed position is not propagated at all — its place comes from an ephemeris, which is what /api/v1/visible answers with."},"slug":"ngc-3998","name":"NGC 3998","class":"lenticular_galaxy_s0","category":"galaxy","object_type":"Lenticular galaxy S0","mag":11.33,"mag_band":"V","abs_mag":null,"abs_mag_band":null,"distance_ly":58933365.356593646,"distance_method":"redshift (CMB frame), flat ΛCDM with H0 = 70 km/s/Mpc","ra_deg":179.483875,"dec_deg":55.453583333333334,"epoch_ra_deg":179.832629,"epoch_dec_deg":55.304067,"epoch_moved_arcsec":null,"pm_ra_mas_yr":null,"pm_dec_mas_yr":null,"orbits":null,"semi_major_axis_km":null,"periapsis_km":null,"mass_kg":null,"mass_msun":null,"constellation":"Ursa Major","discovered":"1789","featured":false,"url":"https://spacecatalog.org/object/ngc-3998","radius_km":null,"radius_kind":null,"data_quality":null,"description":null,"properties":{"b_mag":11.41,"h_mag":7.58,"j_mag":8.33,"k_mag":7.37,"v_mag":11.33,"redshift":0.003646,"hubble_type":"S0","wikidata_id":"Q1108443","redshift_cmb":0.004205547683866495,"discovered_by":"William Herschel","ngc_type_code":"G","uncertainties":{"distance_ly":[14222506.248066716,14222506.248066716]},"major_axis_arcmin":2.78,"minor_axis_arcmin":2.26,"wikipedia_article":"NGC 3998","position_angle_deg":139,"surface_brightness":22.37,"radial_velocity_km_s":1091},"designations":[{"catalog":"2MASX","designation":"2MASX J11575616+5527128"},{"catalog":"IRAS","designation":"IRAS 11553+5543"},{"catalog":"IRAS","designation":"IRAS F11553+5543"},{"catalog":"LEDA","designation":"LEDA 37642"},{"catalog":"MCG","designation":"MCG +09-20-046"},{"catalog":"NGC","designation":"NGC 3998"},{"catalog":"PGC","designation":"PGC 37642"},{"catalog":"UGC","designation":"UGC 6946"}],"source":{"id":"openngc","name":"OpenNGC","url":"https://github.com/mattiaverga/OpenNGC","license":"CC-BY-SA-4.0"},"updated_at":"2026-09-02T19:50:56.720Z","attribution":"Data from SpaceCatalog.org. Individual records carry their source and license in `source`; attribution is required for CC-BY and CC-BY-SA rows."}