Why earth is coldest when it is nearest to Sun in orbit?

It isn't. 'the Northern Hemisphere of Earth is coolest when the angle of the Sun's radiation is more oblique, but the Southern Hemisphere is warmest at the same time, because the angle of the Sun's radiation is more perpendicular at that time. The seasons on Earth are due much more to the angle of the Sun's radiation than the distance from the Sun.

The Earth orbit is not a circle but is slightly elliptical. So there are points in the Earth orbit that are closer and further further from the sun which do effect temperatures. But this heating cooling effect due to proximity is offset by the longer night times that happen during the winter season.

What causes the seasons on Earth is the effect the tilt of the Earths rotational axis. Because of the tilt the duration of the lenght of night change so that in winter the nights are longer. With less day light to warm the ground the temps drop as night time cooling is longer.

The tilt also effects which hemisphere is in a summer time orientation while the other hemisphere is in winter

Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis. In The seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface.

During May, June and July, the northern hemisphere is exposed to more direct sunlight because the hemisphere faces the sun. The same is true of the southern hemisphere in November, December and January. It is the tilt of the Earth that causes the Sun to be higher in the sky during the summer months which increases the solar flux. However, due to seasonal lag, June, July and August are the hottest months in the northern hemisphere and December, January and February are the hottest months in the southern hemisphere.

Earth is NOT coldest when it is nearest the Sun in its orbit. In fact, it is on average about 4 degrees Fahrenheit (2.3 degrees Celsius) warmer than it is in July when it is farthest from the Sun. It is cold only in the northern hemisphere, because in October through February the north pole is tilted away from the Sun: that is the major cause of winter. In these same months, it is summer in the southern hemisphere.

It isn't.

The northern hemisphere is coldest in January (when the Earth is nearest to the sun) because the temperature is controlled by the Earth's tilt not by its distance from the sun.

http://www-spof.gsfc.nasa.gov/stargaze/Sseason.htm

It isn't!

The northern hemisphere is coldest when it's nearest the Sun due to the angle of inclination. The southern hemisphere is at the height of summer in December/January time.

The northern hemisphere is cold because the Sun's rays are spread out over a wider area due to the angle they're hitting us.

In conjunction with tilting, the Earth follows an elliptical orbit, being most distant from the sun during early July. This reduces the intensity of the radiation absorbed from the sun so less heat is accumulated.

Wrong statement! Earth is more cooler when it is away from sun..but thats a mere effect. The main reason for summer and winter is the tilting of earth when its closer to sun!!.

Because of the tilt of the earth,it tilts away from the sun when it is cold and tilts forward when it is summer.

That is not correct. The earth is always the same distance to the sun. But when it becomes winter, the earth tilts away from the sun pushing the bottom hemisphere towards the sun.