PP-Earth-Rotation.zip contains a PowerPoint multimedia presentation on certain concepts of basic astronomy, presented for laypeople. It contains three files: The main PowerPoint file is Earth-Rotation.ppsx There are also two mpeg2 video files that are part of the presentation. After downloading the zipfile, extract all it contents to the same folder. To play the presentation, you need to have appropriate viewing software: Either: Microsoft PowerPoint, version 2007 or later (part of Microsoft Office) or: The free Microsoft PowerPoint Viewer You *MAY* also need to have Microsoft Media Player installed. If so, it is necessary only for viewing the two videos. To download the free Microsoft Viewer, go to the following webpage and follow the links there: http://office.microsoft.com/en-gb/powerpoint-help/view-a-presentation-without-powerpoint-2010-HA102000544.aspx?CTT=1