Explore is a Flickr feature with the intent of showing you "some of the most awesome photos on Flickr." Photos are automatically selected by computer according to a secret algorithm called Interestingness (see below for more about that).

No. It's for photo viewers, not the photographers. It exists so that, at any moment, anyone who wants to view interesting photos can go to Explore and have a reasonable chance of seeing something interesting. Does that imply that photographs not in Explore are uninteresting? Of course not. There are many, many wonderful photos uploaded to Flickr each day that aren't selected for Explore. But to serve its purpose, Explore only has to include a small sampling of all of the photos on Flickr (currently at 500 per day or about 0.005% of the daily upload volume). And Explore tries to show photos from as many different people as possible to create a diverse selection.

Explore is for the viewers. It's a way for Flickr to show the world a sampling of what is being shared there. It's there for those who are new to Flickr, who are lost in the vastness of it all and don't know where to begin. It is not a "best of" listing of photographers. It is not a popularity contest.

Interestingness is what Flickr calls the criteria used for selecting which photos are shown in Explore. All photos are given an Interestingness "score" that can also be used to sort any image search on Flickr. The top 500 photos ranked by Interestingness are shown in Explore. Interestingness rankings are calculated automatically by a secret computer algorithm. The algorithm is often referred to by name as the Interestingness algorithm. Although the algorithm is secret, Flickr has stated that many factors go into calculating Interestingness including: a photo's tags, how many groups the photo is in, views, favorites, where click-throughs are coming from, who comments on a photo and when, and more. The velocity of any of those components is a key factor. For example, getting 20 comments in an hour counts much higher than getting 20 comments in a week.

Here's the application.

Yes. The algorithm is under development and occasionally a new version is deployed. There are two main reasons for this: a) the new algorithm works better and b) it disrupts patterns that allow the same people to feature in Explore day after day. In other words, it shakes things up so that new people can get in and diversify Explore. It's gone through at least three major revisions. The original version only showed something like the top 50. It was later expanded to show the top 500 and the calendar view was added. There were two additional algorithm changes in February and June of 2006.

Scout is a third-party application that uses the Flickr API to retrieve the daily Explore data. It then uses that data to show any user all of his photos that are currently featured somewhere in Explore. Scout is not affiliated with Flickr. I am not a Flickr employee.

Scout updates its database incrementally throughout the day. Scout loads whole days at a time (all 500 photos) back to August 2004 until the entire database is refreshed. And then it starts over. In essence, it takes a snapshot of each day in Explore.

Scout checks each day in Explore for updates and saves the result on a regular basis. It is possible for a photo to enter and then drop out of Explore before Scout has a chance to scan that date. If that happens, and the photo never reappears in Explore, Scout will have no record of it.

That's rare though because Scout scans Explore so frequently.

Maybe someone saw your photo on the Explore front page. But Explore for any given day doesn't become "official" until the following day at midnight (Pacific Time).

There's a reason Flickr doesn't link to the current day's Explore (only yesterday's and older). The results don't become "official" until the day ends when all of the photos for that day can be evaluated fairly. Otherwise it's like running a race but letting some people start early. Rankings don't "count" until the very last photo of the day is uploaded (because that one could be #1 for the day).

The Flickr API, which is what Scout uses to get Explore data, doesn't even give valid data for the current day. So Scout waits until midnight when the results become official.

Days in Explore are counted using PST (GMT-8) because that's Flickr time.

No. The rankings in Explore for all dates change all the time even for dates in the past. Photos are uploaded to Flickr at a rate of around 5,000/minute and are constantly being viewed, faved, and commented by millions of members. Interestingness rankings are recalculated for all photos many times per day. Photos uploaded a day, a week, even a year ago, are still receiving activity in the form of views, comments, favorites, etc. And all of that activity affects each photo's Interestingness ranking, even for photos that were uploaded years ago. Explore is very dynamic. When you browse through Explore on any date, you are viewing what the Interestingness algorithm has selected as the highest ranked photos at that moment.

Rankings are relative to other photos uploaded on the same day. If everyone else moves up, you move down.

Were they uploaded on different days? Remember, to get into Explore every photo has to "compete" against every other photo on Flickr; and every day is a different competition with different photographs. You can't compare rankings from day to day. That is, the #1 photo in Explore today might have been #497 (or might not even have made the cut) if it was uploaded yesterday—even though in your personal listing it is your highest ranked photo.

It seems counter intuitive but it makes sense. Interestingness is not calculated in real-time, microsecond to microsecond. It is calculated en masse at intervals, perhaps once per hour. Imagine all of the activity that occurs in an hour—not just on your photo but all of the other photos in Explore and on the rest of Flickr. All of that activity goes into the Interestingness algorithm and every photo is re-ranked. The result of that activity is that many photos' Interestingness rankings may have changed drastically.

No. If it were completely random then you'd never see any stability in a photo's ranking and there are many photos that have maintained the same position for months on end.

No one knows for sure except Flickr staff. Many people speculate that it is a random factor in Explore. That may be.

Mainly it's because there are really more than 500 photos in Explore each day (around 2000 typically) and they rotate through the top 500 spotlight.

Also, as time goes on, old photos keep getting new activity so photos are added and dropped from old dates in Explore all the time.

If a photo is dropped, it's because it wasn't in Explore when Scout scanned that day.

Scout started keeping a permanent record of every photo that was ever in Explore on May 24th 2007. Before that, Scout would delete dropped photos from the listings after a few days. You're out of luck if your photo dropped before May 24th.

Because Scout and Explore are updated at different intervals, the rankings may be slightly out of sync. There's a more detailed answer two answers down.

Because Scout and Explore are updated at different intervals, it's possible for Scout to miss a photo if it enters and then drops out of Explore very quickly. There's a more detailed answer below.

There are a lot of factors at work. Take this theoretical sequence of events:

1. Scout and Explore are synchronized.
2. Flickr updates Interestingness and the photo drops. At this point, Scout will still show the photo is in Explore when it is not.
3. Scout updates and marks the photo dropped. Now Scout and Interestingness are in sync again.
4. Interestingness updates and the photo comes back into Explore. At this point, Scout will show that the photo has dropped even though it is currently in Explore.
5. Scout updates and marks the photo added. And now Scout and Explore are back in sync again.

If you check Scout and Explore before step 1 and again at Step 4 or 5 you may get the impression that the photo has never left the Explore pages when in reality it did for a brief period of time.

It's actually more complicated than that because there's a huge website caching mechanism at work. And Scout doesn't just update it's entire database every few hours. It updates recent days more frequently and updates the database incrementally rather than all at once.

The bottom line is this: if Scout says a photo dropped, then Flickr told Scout it wasn't in Explore at the exact moment that Scout asked.

This happens when you change the privacy on a photo or replace it with a new photo or delete a photo. If you make a photo private, it drops out of Explore and Scout but because it is private the thumbnail can no longer be viewed. When you replace a photo the photo ID is changed and the thumbnail is temporarily unavailable until the next update. In some cases, replacing a photo also causes it to drop out of Explore in which case it may never show up again.

You can prune these from Scout by linking your account, viewing Dropped photos, and then clicking Remove. Photos pruned in this way will reappear if they come back into Explore.

Photos can appear on multiple days if you change the date of the photo after it has been in Explore.

You can prune these from Scout by linking your account, viewing Dropped photos, and then clicking Remove. Photos pruned in this way will reappear if they come back into Explore.

Since photos dropped from Explore don't have a ranking, Scout shows the last known ranking at the time it dropped.

DNA shows photos that are currently in Explore as well as photos that used to be in Explore but are no longer there.

Scout, by default, only shows photos that are currently in Explore. Click "Include dropped" on the Scout page to include photos that used to be in Explore.

Scout keeps a 10 day history of rankings and also tracks the highest ranking achieved for all time. Click the position change indicators (red and green arrows or black dots next to the rank) to view the history for a photo.

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.

Scout tracks every day back to August 2004. This is every date that Explore tracks.

Views and faves are only part of the Interestingness ranking. The speed at which those views/faves occurred and many other factors go into calculating Interestingness.

Yes, that's true.

Yes, that's true (as of October 2006).

Explore always counts days in Pacific Time for the purpose of displaying photos on the Explore calendar. That is, the start of each day as counted in Explore is at midnight Pacific time or GMT-8.

There are several ways. You could delete it. Another way is to make the photo private. Only public photos are featured in Explore. You can also click the "Hide this photo from public searches" link on one of your own photos. When you do that, it tells Flickr that you don't want that photo shown in public areas.

No, not really. I am not affiliated with or employed by Flickr. If you're upset that your photos aren't in Explore, the best advice I can give you is this: convince yourself that it doesn't matter. Because it doesn't.

Pandas.