barycentric coordinate time (tcb) is the analog of tcg, used for calculations relating to the solar system beyond earth orbit. tcg is defined by a different reference frame from tcb, such that they are not linearly related. over the long term, tcg ticks more slowly than tcb by about 1.6 × 10 (about 0.5 seconds per year). in addition there are periodic variations, as earth moves within the solar system. when the earth is at perihelion in january, tcg ticks even more slowly than it does on average, due to gravitational time dilation from being deeper in the sun's gravity well and also velocity time dilation from moving faster relative to the sun. at aphelion in july the opposite holds, with tcg ticking faster than it does on average.