Why are really old photos in Explore?
Remember that photos are in Explore because their Interestingness is high enough now, today, at this moment. Explore shows photos that currently have a high Interestingness score and Interestingness is recalculated constantly for all photos regardless of when they were uploaded. The date a photo was uploaded determines the box it occupies on the calendar. A photo uploaded in 2005 could have been Interesting enough to be in Explore in 2005 but later dropped. Months, even years later, new activity on that photo could drive it’s Interestingness up enough to show it in Explore again.
