If you're looking to download the Photo Ephemeris Web, well... you can't.
The app truly is a web app and it runs in your browser. Just visit https://app.photoephemeris.com in your browser to get started. If you try to download it via File, Save As... or to save it as a .mht file (Web page archive file format), it will not work.
Make sure you're running a recent version (i.e., updated within the past 6 months) of a major web browser. Safari, Chrome, and Firefox are preferred.
Make sure your browser is configured to allow Cookies and Local Storage for websites (or at least for *.photoephemeris.com).
If you are having problems accessing the app, check that you don't have security software running that is blocking access to Photo Ephemeris Web, as this happens occasionally.
If you'd like a to create a bookmark or desktop shortcut to access the web app, check out the following help article: How can I create a desktop shortcut or bookmark?