The motto has been variously translated over the years. A good motto will rarely bear literal translation, more important is the general sense and implication of the sentiment it expresses or the admonition it conveys.
If a literal translation is required, then "The constellation is changed, the disposition is the same" is perhaps appropriate. The ablative absolute in Latin can be used in place of a number of other constructions. Here it probably has a concessive force: "though the constellation is changed..." sidus means primarily in Latin 'a group of stars,' 'a constellation'. To translate simply 'star,' as many of the suggested translations do, is incorrect. Again, mens in Latin has a much wider range of meanings than 'mind,' 'the mental functioning of human animals,' here the sense is clearly disposition, e.g. towards learning and scholarship.
Hence it is easy to arrive at the general sense: "The traditions of the older universities of the Northern Hemisphere are continued here in the Southern."