Today the Earth is closest to the Sun, a point called perihelion. We will be about 147,099,000km (91,403,000miles) from the Sun. Compare this to the 152,096,000km (94,508,000miles) we will be at aphelion on July 4th, a difference of about 4,996,000km (3,104,000miles) occurs throughout one orbit.
It may seem odd that we are actually at the closest for the middle of northern winter, you just have to remember that proximity to the Sun is not the cause of the seasons. The seasons are caused by the axial tilt of the Earth, creating short and long days throughout the year, with a resulting change in the angle and intensity of the sunlight.
| 2013 Solstices and Equinoxes | ||||
|---|---|---|---|---|
| UT | HST | |||
| Perihelion | Jan 2 | 00:59UT | Jan 1 | 14:59HST |
| Spring Equinox | Mar 20 | 11:02UT | Mar 20 | 01:02HST |
| Summer Solstice | Jun 21 | 05:04UT | Jun 20 | 19:04HST |
| Apehelion | Jul 5 | 18:59UT | Jul 5 | 08:59HST |
| Fall Equinox | Sep 22 | 20:44UT | Sep 22 | 10:44HST |
| Winter Solstice | Dec 21 | 17:11UT | Dec 21 | 07:11HST |
| Source: NASA Sky Calendar | ||||
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While the seasons are a mainly a result of the tilt of the Earth relative to the sun, it is worth noting there is still an appreciable effect upon the amount of Solar radiation intercepted as a function of our position in orbit.
From apehelion to perehelion the light intercepted varies from 1414 W/sqM to 1323 W/sqM. Nothing to sneeze at. So the majority of humanity, in the Northern Hemisphere, is suffering reduced light as a function of angle but increased light as a function of proximity. Meanwhile some greedy buggers in the Southern Hemisphere are enjoying both the angle and the icing on their geospatial cake, proximity to the Sun!
Futhermore, another notch lower in scale, the output of the sun itself is variable.