  Cepheids play a key role in astronomy as standard candles for measuring
intergalactic distances. Their distance is usually inferred from the
Period-Luminosity relationship, calibrated using the semi-empirical
Baade-Wesselink method. Using this method, the distance is known to a
multiplicative factor, called the projection factor. Presently, this factor is
computed using numerical models - it has hitherto never been measured directly.
Based on our new interferometric measurements obtained with the CHARA Array and
the already published parallax, we present a geometrical measurement of the
projection factor of a Cepheid, delta Cep. The value we determined, p =
1.27$\pm$0.06, confirms the generally adopted value of p = 1.36 within 1.5
sigmas. Our value is in line with recent theoretical predictions of Nardetto et
al. (2004).
