Astronomy is a VERY small department (by student count) at Cornell. If you have a genuine interest in the subject (backed up by a history of activity in the field), you should reach out to the department and see if you can speak to or email a Professor. Ask some relevant and insightful questions to determine if it is the right major for you. You very well may develop an advocate that can help you in admissions. It is possible that you would be considered “unique” as an applicant, as well.
Cornell Fuertes observatory is cool, and the Cornell Astronomical Society is always looking for members.
On the other hand, the CS major (between A&S and Engineering) has a huge number of students.
There are a few interesting classes being offered this coming spring that will bring your two interests together:
ASTRO 3334Data Analysis and Research Techniques in Astronomy
ASTRO 3340Symbolic and Numerical Computing
I can’t see why you couldn’t write a compelling supplemental essay to demonstrate your interest in both 