Sunshine and Temperature

A bit of a ontinuation from what I did before on the amount of sunlight a location receives. I've always thought that the amount of daylight a location got was an indicator of temperature.  The more sunlight, the more energy imparted to the earth, the warmer the place would be.  The question that has been bothering me is why the shortest day of the year (winter solstice) is the not the coldest day of the year.  I found the average temperature from 1971 to 2000 from NOAA, and used the location of Philly for the sunlight.

 I then see that the temperature seems to follow the sunlight with a difference in peak position of about a month.  This fits in with the experience that the end of January is usually the coldest, a month after the shortest day.  The conclusion I got for this is that the earth is a much bigger heat sink than what I previously thought - it takes about a month to cool it down (late fall), and about a month to heat it back up (early spring).