Letter, from John Flamsteed to Henry Oldenburg, dated at Derby Discussing Cassini's method for observing the diameter of Jupiter and parallax of Mars; regrets that Cassini's letters did not cover the meridional altitudes of the sun; criticisms about the letter which discussed the moon's right ascension from the sun for the meridian of Paris; suggests observations of the moon's diameter with discussion of Horrocks's theory on this topic 24 August 1673