Diagram. A sort of shorthand picture of an apparatus or part of an apparatus. It is used to show in the simplest and clearest manner the construction of an apparatus.
Diameter. [See Appendix B].
Direct line. A straight line.
Disk. (1) A circular plane or flat surface. Since the Sun, Moon and planets seem to be flat their surfaces are called disks. (2) Flat and circular like a coin.
Dog days. So called from the fact that Sirius, the Dog Star, rises at the same time as the Sun.
Ecliptic. [See Appendix B].
Elastic. (1) A body that will stretch and return to its original size. (2) A thin piece of rubber.
Ellipse. [See Appendix B].
Equator. [See Appendix B].
Equatorial. Having to do with or affected by the equator.