The value of a in an elliptic orbit is known in astronomy as the semi-major axis and it is regarded as one of the six orbital elements which define the motion according to Kepler's laws . The foci of an ellipse have an interesting property. An ellipsoid of revolution is the 3-dimensional figure obtained by rotating an ellipse around one of its axes. If a hollow ellipsoid of this shape is made and its inner surface is silvered to act as a mirror , then if a source of light is placed at one focus, all of its rays will converge at the other one. Even if only part of the ellipsoid is silvered, all light hitting that part will still be concentrated at the other focus.
Sound waves can behave like light, too.
This was done for architectural reasons, nearly 200 years ago, but it also enabled a person at one focus to overhear anything spoken at the other focus, even whispers. Supposedly, Daniel Webster sat at one such spot and made good use of its special nature. Today the House of Representatives has many more members, uses a much bigger chamber and its old meeting room is a museum displaying statues of distinguished Americans. Every year many thousands of visitors are guided through that chamber. By the way, a well-known painting of that chamber, with faces of its occupants clearly identifiable, was made by Samuel Morse, the artist who also invented the electric telegraph.