On the 4th of July Earth will be at its greatest distance from the Sun — an
annual event called “aphelion.”

On the 4th of July Earth will be at its greatest distance from
the Sun this year. But don’t expect any sudden relief from the heat, say
scientists. Northern summer will continue unabated –perhaps even worse than
usual– despite our arrival at a distant part of Earth’s orbit called
“aphelion.”

“Like all planets in our solar system, Earth travels around the Sun in an
elliptical orbit,” explains astronomy professor George Lebo, a Summer
Faculty Fellow at the NASA Marshall Space Flight Center. “We make our
closest approach to the Sun (147.5 million km) in January, that’s called
perihelion, and we’re farthest from the Sun (152.6 million km) in July,
that’s aphelion. This year, aphelion falls on Independence Day in the USA.”

The eccentricity of our planet’s orbit is mild; aphelion and perihelion
differ from the mean Sun-Earth distance by less than 2%. In fact, if you
drew Earth’s orbit on a sheet of paper it would be difficult to distinguish
from a perfect circle. “Seasonal weather patterns are shaped primarily by
the 23.5 degree tilt of our planet’s spin axis, not by the mild eccentricity
of Earth’s orbit,” notes Lebo. “During northern summer the north pole is
tilted toward the Sun. Days are long and the Sun is shining more nearly
straight down — that’s what makes July so warm.”

Does aphelion bring any relief from northern heat? Yes and no, answers Roy
Spencer of the Global Hydrology and Climate Center in Huntsville, Alabama.

“Averaged over the globe, sunlight falling on Earth in July (aphelion) is
indeed about 7% less intense than it is in January (perihelion).” That’s the
good news. The bad news is it’s still hot. “In fact,” says Spencer, “the
average temperature of Earth at aphelion is about 4o F (2.3o C) higher than
it is at perihelion.” Earth is actually warmer when we’re farther from the
Sun!

How can that be? It’s because our planet is –in a sense– lopsided.
Continents and oceans aren’t distributed evenly around the globe. There’s
more land in the northern hemisphere and more water in the south. During the
month of July –near the start of northern summer– the land-crowded
northern half of our planet is tilted toward the Sun. “Earth’s temperature
(averaged over the entire globe) is slightly higher in July because the Sun
is shining down on all that land, which heats up rather easily,” says
Spencer.

Solar heating raises the temperature of continents more than water because
the heat capacity of land is lower. Bill Patzert, an oceanographer at NASA’s
Jet Propulsion Laboratory explains: “The temperature of land changes easily,
which is what we mean by low heat capacity. Consider the desert: At night
the desert is cold, perhaps only 60o F (16o C). When the Sun rises in the
morning the temperature might jump to 100o F (38o C) or more.” Such
mercurial behavior is characteristic of materials like rocks and soil with
little thermal inertia. It doesn’t take much sunlight to substantially
elevate their temperature.

“On the other hand,” says Patzert, “oceans have a tremendous heat capacity.
They tend to keep [the energy] they capture and are very stingy about giving
it back.” This quality moderates the temperature swings of ocean
environments. “Let’s say you went sailing off Malibu Beach at noon,” says
Patzert. “The offshore temperature might be 75o F (24o C) — pretty
pleasant!” What happens after sunset? “The temperature drops, but only a few
degrees because the thermal inertia of the ocean is so high.”

All this explains why July is our planet’s warmest month: Northern
continents baked by the aphelion Sun elevate the average temperature of the
entire globe. Six months later, in January, the situation is reversed as our
planet presents its water-dominated hemisphere to the Sun. “We’re closer to
the Sun in January,” says Spencer, “but the extra sunlight gets spread
throughout the oceans.” Southern summer in January (perihelion) is therefore
cooler than northern summer in July (aphelion). Strange but true!

“Another notable difference between summers in the two hemispheres is their
duration,” adds Lebo. According to Kepler’s 2nd Law, planets move more
slowly at aphelion than they do at perihelion. As a result, Northern summer
on Earth is 2 to 3 days longer than southern summer — which gives the Sun
even more time to bake the northern continents.

If you’re feeling baked on the 4th of July holiday and wish that aphelion
brought more relief, there is something you can do: Take a hint from the
watery southern hemisphere. Locate the nearest swimming pool and sample
first hand the benefits of water’s high heat capacity. A little physics can
be refreshing after all!