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Actually, Zack Greinke is a bit of a day-game specialist for Arizona this season: maybe we should start calling him “daywalker”. This will already be his sixth start in a day game, compared to only four after the sun has gone down. No-one else on the club has more than three day starts to this point, though the team does seem to have been playing a lot of them so far. This will be day game #17 for the 2019 Diamondbacks, putting them on pace for 63 such games this seaason: last year, there were only 47 such in 2018. [In case you’re wondering, the 5:10 pm Saturday starts are classed as night games] So far, they’ve been a bit of a problem, the D-backs going 7-9 in day games, compared to 16-11 at night.
This does actually seem to be something of a trend for the D-backs. Over their entire history, Arizona are slightly above .500 in night games, with a record of 1230-1214, a win percentage of .503. But in day contests, they are only 471-530, a W% which is thirty-three points less. That may seem little or nothing, but over a thousand-game sample, it’s not insignificant. If we assume the D-backs “true” talent is their overall .494 win percentage, the chances of getting a .470 W% or worse by chance, over 1004 games, is only 6%. This suggests the Diamondbacks do actually play a bit worse during day games.
Digging a bit further, we find the Arizona ERA during the day is one-tenth of a run higher at 4.29, compared to 4.19 at night. This is true for the NL overall, but to a smaller degree, with day ERA over the same 1998-2019 timeframe being 0.04 higher (4.21 vs. 4.17). This feels in line with what I’d intuitively expect: “natural” lighting conditions to favor hitters, and they generally seem to do slightly worse under artificial light. However, Arizona’s hitters don’t follow suit. Our OPS in night games is +14 to day (.747 to .733), while the overall NL split is almost nothing (.738 vs. .739). It seems that this slight difficulty hitting in the day is perhaps the main cause of our overall below-average record there.