Our greatest distance of the year from the sun, but a hot week ahead
Posted July 6, 2015
At 3:41 PM today (Monday afternoon, July 6th) the Earth reach "aphelion," its greatest distance from the sun of the year. We'll be about 94.5 million miles away from the sun at that point, compared to 91.4 million miles when we were at our closest approach ("perihelion") of the year back on January 4th. As we travel along our slightly elliptical orbit, this eccentricity leaves us about 3 percent farther away, the sun about 3 percent smaller as seen from here, and about 7 percent less solar radiation falling upon the entire earth, as compared to back in early January.
Of course, this is summer and it's hot out there, and the reason for this being our hot time of the year is not related to how far away we are from the sun, but the direction in which the earth's rotational axis is tilted. This time of year, the north pole is tiled strongly (over 23 degrees) toward the sun, leaving the sun at a much higher angle in the sky than it reaches during the northern hemisphere winter. This concentrates more of the sun's energy into a given area, heating the surface more strongly, whereas the lower angle of the sun in winter spreads the same amount of energy over a larger area for a considerably lower rate of heating. In addition, of course, that change of tilt also leaves us with a much longer time between sunrise and the following sunset this time of year, so we receive those rays of sunlight for longer periods, and conversely lose heat by way of radiation from the surface for a shorter time at night.
These tilt-related effects are overwhelmingly stronger than the impact of our varying distance from the sun, and leave us with our typical summer heat and winter cold. However, if the timing of perihelion and aphelion were reversed, it is likely that our winters would be a little colder than they are, and our summers a little hotter.
For the past week or so, our temperatures have moderated a good bit compared to some of the heat we experienced in June (some of us had highs in the 70s last Friday!). However, it does appear a building ridge aloft will bring us a round of hotter weather for much of this week and possibly through the weekend. So far, it doesn't look quite as hot as that one stretch a few weeks ago, as highs this week will likely reach into the mid 90s for a few days, while cooling showers or thunderstorms, while not out of the question, become rather few and far between.
For those of you interested in keeping track of when aphelion or perihelion occurs, I've included a link to the U.S. Naval Observatory's data services page. Look under "Dates and Times" there for the "Earth's Seasons..." link.