What is the declination of Earth?
The declination of Earth is 0° by definition.
The positions of stars are defined in terms of right ascension and declination.
Right ascension is the angle the object makes with the Vernal Equinox at a particular epoch, usually J2000. The reason for this is that the Vernal Equinox precesses and a fixed frame of reference is required.
Declination is the angle the object makes with the Earth's Equator. Again this requires a fixed frame of reference such as J2000 due to precession.
The J2000 epoch is the position of the Earth at exactly 2000-01-01 12:00:00.
By definition the Earth is at the centre of the coordinate system and has a declination of 0°.