General rule of thumb for gardening is at least 6 hours of sun in your case on Dec 21 at least some of it afternoon sun for tomatoes. Anything less may or may not work. If you are gardening against a south wall with less sun then they will do better than out in the middle of the yard. If you are new to gardening all you can do it ride this year out an see what happens with the best you can do. There are always things like cilantro, lettuce and parsley that will do well with less daylight hours.