Outdated Links to Dynamic Images

OUTDATED IMAGE LINK You probably arrived here after having seen the image at the right appear in the middle of a document at some other site.

The www.fourmilab.ch site hosts a variety of resources: Earth and Moon Viewer, Solar System Live, and Your Sky, which permit you to create custom images of the Earth, Moon, Solar System, and celestial objects. Every image is created to requesters' specifications and returned via a results page which includes the dynamically generated image.

At the current request rate, between three and ten unique images are generated every second; all the disc space in the world wouldn't permit keeping them forever. So, these images are kept in a “cache” directory which is periodically purged of images older than ten minutes. This provides plenty of time for users to see the images they request, but expunges them before they bury the server in detritus.

All of this is well and good unless users try to embed these dynamically-generated images into their own Web pages (or include links to them in message board postings or E-mail, etc.). If the user tests the link while the image still happens to be in the temporary cache directory, it will work, but when somebody subsequently tries to follow the link after the dynamic image has been purged, they'll get a “broken image” result.

To avoid confusion when this happens, we redirect requests for nonexistent images to the “Outdated Image Link” image which brought you here. Whenever you see this image, it indicates somebody has linked to one of our dynamically generated images which has since been deleted.

It's easy to avoid this problem. Rather than linking to the image, simply save it to your own computer, then upload it to your own Web site and reference it from there. If you'd like to make a back link to www.fourmilab.ch that's much appreciated but not at all required. (Earth Viewer images derived from The Living Earth image database are subject to their copyright and may not be reproduced without permission; please contact The Living Earth for details. All other images generated by Fourmilab Web resources are in the public domain and may be used in any manner without permission or any restrictions.)

If you want to link to a dynamically-updated image (for example, showing the sky at the present time above a given location, or current clouds from weather satellites), create a custom request and link to the resulting URL from your site.

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by John Walker
April 22nd, 2003