heliometry

from The Century Dictionary.

• noun The art of using the heliometer; also (rarely), the art of making measurements upon the sun.

from the GNU version of the Collaborative International Dictionary of English.

• noun The apart or practice of measuring the diameters of heavenly bodies, their relative distances, etc. See heliometer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The measurement of the diameters of heavenly bodies, their relative distances, etc.