• A directed message intended for our receipt.

This is the best (easiest) case. The 300-m radio telescope at Arecibo, Puerto Rico (see Fig. 20.7, page 499) could beam a message to a like telescope anywhere in the Milky Way Galaxy.

• Eavesdropping on general internal communications.

We have been announcing our existence to the rest of the Galaxy for the last 65 years or so by commercial radio and television broadcasts that transmit outwards at the speed of light (out to the closest 3000 stars).

These signals will have reached the nearest million stars within 400 years.

An extraterrestrial civilization could deduce that our planet spins once every 24 hours (by the fluctuation in signal strength with rotation) and orbits the Sun in 365 days (by measuring the Doppler shifts of the radio signals).