Precision CCD photometry on the BV system to V≅20 is presented and analyzed for the old open cluster NGC 2420. The unevolved main sequence is shown to be narrow and well defined to the completeness limit of V = 18.5. The distribution of stars away from the main sequence is shown to be bimodal, with a secondary peak located approximately 0.7 mag above the fiducial main sequence. Adopting the bimodal mass ratio distribution of Trimble (1974) leads to an estimate that 50% of the cluster systems are binary. Near the turnoff, the cluster sequence exhibits some detailed structure. The fact that the turnoff broadens where the binary sequence crosses the turnoff implies a causal relationship, but is inconsistent with standard composite evolution of equal-mass systems. More significantly, the turnoff region contains a definite red hook and a populated subgiant branch which is best explained by models that include convective overshoot mixing. Fiducial relations are given for the cluster extending from the lower main sequence to the red giant branch 1.5 mag above the clump. Adopting E(B -V) = 0.05 and [Fe/H] = -0.4 for NGC 2420, comparisons are made between the cluster and NGC 2506, the isochrones of VandenBerg (1985), and 47 Tuc. Assuming NGC 2506 has the same metallicity as NGC 2420 leads to E(B - V) = 0.06 and an apparent modulus that must be larger by 0.45 mag than that for NGC 2420. Surprisingly, though NGC 2506 is found to be younger, alignment of the two clusters places its red giant clump 0.2 mag fainter and slightly bluer than that for NGC 2420. Adopting a solar (B - V) of 0.65, the adjusted isochrones of VandenBerg (1985) produce an age of (3.4 ± 0.6) × 109 yr, where the large uncertainty arises because of the inability of the standard isochrones to match the shape of the cluster turnoff. On a scale where (m - M) = 9.6 for M67, NGC 2420 has an apparent modulus of (m - M) = 11.95. This implies a spread in the location of the red giant clump among old open clusters of at least 0.3 mag. Finally, comparison with 47 Tuc under the assumption that its metallicity is lower than that for NGC 2420 by 0.4 dex leads to an apparent modulus of 13.4 on the scale above. Using the relative position of the red giant branches and the luminosity of the clump, the metallicity is constrained to lie between [M/H] = -1.0 and - 0.6, assuming both clusters have the same helium abundance. Some suggestions for future work are discussed.
