However, for these characters to appear, the packages wasysym and marvosym need to be present and there doesn't seem to be any easy way to get these into the Blogger template. They are already present in MacTeX and it is simply a question of loading the packages using the usepackage command before starting a document. However, the packages must be loaded after the amsmath package.
For the moment, it would seem that the special characters feature in Blogger is the only way to access astrological symbols. Well, that's not entirely true. The special characters features is just a convenient way to insert the appropriate alt code into the HTML e.g. the alt code for Mercury is ☿ and it displays thus ☿. You can do this by simply going to a table of alt codes, copying the code and pasting it into the HTML of your blog. Easier of course just to use the special characters feature. A complete list of all possible characters, not just astrological one, can be found here.
