Why is the earth's orbital velocity is slowest on July 5?

Answer 1

it is farthest from the sun. As the earth moves around the sun it moves in an eliptical orbit as it gets closer it speeds up and as it moves away it slows down.

Answer 2

Because that is when we are the furthest from the Sun (a point on the orbit called aphelion).

The closest point is called perihelion (January) and that is when our orbital speed is fastest.

Orbital speed depends on the mass of the central body (the more massive, the faster the speed) and the distance from the central body to the orbiting one (the closer, the faster).

In our case, it is the same Sun (mass does not change over the year), so the only difference is distance.

Closer means more gravitational attraction
F = G M m / d^2
F is the force
G is a constant
M is mass of sun
m is mass of Earth
d is distance (d^2 is distance squared)

To keep the Earth on its orbit, an equal amount of centrifugal ('fleeing the centre') force is needed to balance the gravitational (centripetal) force. Centrifugal force depends on the orbital speed. SInce there is less centripetal force in July, we need less centrifugal force to balance it -- therefore we go slower.

Answer 3

Kepler's laws
Law of Ellipses (1609). The orbit of each planet is an ellipse, with the Sun located at one focus. Earth is closest to the Sun in January and farthest from the Sun in July as it travels along its elliptical orbit.
2)Law of Equal Areas (1609). A line drawn from the planet to the Sun sweeps out equal areas in equal times. This geometric description captures the fact that a planet's orbital velocity varies in a regular way -- the farther the planet is from the Sun, the more slowly it moves along its orbit. Earth moves fastest in January, and slowest in July.
3)Harmonic Law (1618). The square of the sidereal period of a planet is directly proportional to the cube of the semimajor axis of its orbit. In non-technical terms, planets move more slowly, and in a predictable way, the greater their distance from the Sun. Mercury speeds once around the Sun in just 88 days, Earth takes one year, Mars almost two years and distant Pluto almost 250 years.

Answer 4

The Earth reaches aphelion (the farthest point from the Sun) in its orbit around that date. Consevation of angular momentum implies that it would travel slower at the point due to the longer moment arm.
BTW, the reason why it ends up being hotter in the Northern Hemisphere during that time is due to the Earth's tilt relative to its orbit and not necessarily the distance to the Sun.