Circular orbit  A circular orbit is the orbit with a fixed distance around the barycenter, that is, in the shape of a circle. Listed below is a circular orbit in astrodynamics or celestial mechanics under standard assumptions. Here the centripetal force is the gravitational force, and the axis mentioned above is the line through the center of the central mass perpendicular to the plane of motion. In this case, not only the distance, but also the speed, angular speed, potential and kinetic energy are constant. There is no periapsis or apoapsis. This orbit has no radial version. Transverse acceleration (perpendicular to velocity) causes change in direction. If it is constant in magnitude and changing in direction with the velocity, circular motion ensues. Taking two derivatives of the particle's coordinates with respect to time gives the centripetal acceleration  where: The formula is dimensionless, describing a ratio true for all units of measure applied uniformly across the formula. If the numerical value of formula_5 is measured in meters per second per second, then the numerical values for formula_2 will be in meters per second, formula_3 in meters, and formula_4 in radians per second. The relative velocity is constant: where:  The orbit equation in polar coordinates, which in general gives "r" in terms of "θ", reduces to: where:  This is because formula_16  Hence the orbital period (formula_18) can be computed as:  Compare two proportional quantities, the free-fall time (time to fall to a point mass from rest)  and the time to fall to a point mass in a radial parabolic orbit  The fact that the formulas only differ by a constant factor is a priori clear from dimensional analysis. The specific orbital energy (formula_22) is negative, and  Thus the virial theorem applies even without taking a time-average:   The escape velocity from any distance is times the speed in a circular orbit at that distance: the kinetic energy is twice as much, hence the total energy is zero. Maneuvering into a large circular orbit, e.g. a geostationary orbit, requires a larger delta-v than an escape orbit, although the latter implies getting arbitrarily far away and having more energy than needed for the orbital speed of the circular orbit. It is also a matter of maneuvering into the orbit. See also Hohmann transfer orbit. In Schwarzschild metric, the orbital velocity for a circular orbit with radius formula_25 is given by the following formula: where formula_27 is the Schwarzschild radius of the central body.