The windier the conditions, the more the stars’ light would smear across what is called an arc second. And to find Planet Nine they needed all arc second readings to be under 2.0, ideally under 1.0. Planet Nine likely travels — at the most — two arc seconds a night, so if the winds are too high in the upper atmosphere, so much that it’s smearing the stars into two or three arc seconds wide, the data become unusable. Think of zero arc seconds as being a perfect point in the sky; as the arc seconds creep up, the light gets blurrier, smearing out a little to the sides and blocking whatever possible planet might be hiding behind.
Brown named each field with four numbers in a spreadsheet and kept a log of stars’ arc seconds that Batygin randomly clicked on in that field. If the “seeing” was bad, Brown would make a note in the log and they would have to go back and reimage that field. This is where observing becomes less romantic and more like a creepy radio number station. They would wait to take about 10 images, and Batygin would then read off the numbers in batches: “4817 is 1.4. 4918 is 0.9. 4919 is 1.05. 5319 is 1.1. 5318 is 1.4,” and so on.