We all know that Earth orbits our sun in an elliptical path, which means that for certain times of the year it is closer to the sun than for others. Here is a distorted view of the basic orbit (Credit: Wikipedia), but beware that the scales are all wrong. There is only a 5 million kilometer difference between the longest and shortest lengths of the ellipse!
According to the 1996 US Ephemeris, on the 21st of each month, the distance to the Sun is:
Month...............distance............ January 0.9840 147,200,000 February 0.9888 147,900,000 March 0.9962 149,000,000 April 1.0050 150,300,000 May 1.0122 151,400,000 June 1.0163 152,000,000 July 1.0161 152,000,000 August 1.0116 151,300,000 September 1.0039 150,200,000 October 0.9954 148,900,000 November 0.9878 147,700,000 December 0.9837 147,200,000 .................................
where the first column gives the distance in Astronomical Units so that 1.0 AU = 149,597,900 kilometers By the way, I have rounded all of the distance numbers to 4 significant figures. If you want a mathematical formula that gives you the distances in AU using these table entries for month number N, it looks like this:
d(N) = 1.0000 + 0.0163 cos [ (2pi/12)*((N-1)/12) + 200*2pi/360]
As you can see from the table, the Earth is farthest from the Sun when the Northern Hemisphere is in the summer season (July 3), and closest in the winter (January 3). If you were living in the Southern Hemisphere, the seasons are reversed so that in the summer, the Sun is closest and in the winter it is farthest.