The sun is considered “directly overhead” when its rays strike the surface at a perfect 90-degree angle, a phenomenon known as the subsolar point. At this precise moment, a vertical object, like a flagpole, casts no observable shadow. For the vast majority of people in the United States, the sun never reaches this absolute zenith position during the year. This is a geographical certainty because the continental United States (the lower 48 states) is situated too far north for the subsolar point to ever pass over it.
What It Means for the Sun to Be Directly Overhead
The location where the sun is directly overhead is governed by Earth’s \(23.5^{\circ}\) axial tilt. As the Earth orbits the sun, this tilt causes the angle of the sun’s most direct rays to shift between a northern and a southern limit, defined by the two major circles of latitude known as the Tropics.
The northern boundary is the Tropic of Cancer, and the southern boundary is the Tropic of Capricorn. The sun can only achieve a \(90^{\circ}\) angle above the horizon for locations situated between these two lines. This movement is known as the change in solar declination, which dictates the latitude of the sun’s most intense light.
The Tropic of Cancer, located at approximately \(23.5^{\circ}\) North latitude, marks the northernmost limit where the sun can be directly overhead. On the Summer Solstice, typically around June 21st, the sun reaches its maximum northern declination, sitting directly over the Tropic of Cancer. It then begins its apparent journey southward toward the equator and eventually to the Tropic of Capricorn.
Why the Continental US Falls Short
The reason the sun is never directly overhead in the continental United States is geographical: every part of the lower 48 states lies north of the Tropic of Cancer. To experience the subsolar point, a location must be at or south of this line, which is situated at about \(23^{\circ}26′\) North latitude.
The absolute southernmost point of the contiguous United States is in the Florida Keys, near Key West, at approximately \(24.5^{\circ}\) North latitude. This means the southernmost edge of the continental US is still about 69 miles north of the Tropic of Cancer.
Since the sun’s maximum northern position is fixed at the Tropic of Cancer, the required \(90^{\circ}\) angle is geographically unattainable for the continental U.S. The only U.S. state that experiences the sun directly overhead is Hawaii, which straddles the tropical zone.
The Highest Sun Angle of the Year
Although the sun never reaches the zenith, it reaches its highest point in the sky for the year on the Summer Solstice, around June 21st. This annual maximum angle occurs because the Northern Hemisphere is tilted most directly toward the sun at this time.
The maximum solar angle above the horizon can be calculated by comparing a location’s latitude to the sun’s maximum declination of \(23.5^{\circ}\) North. At the southernmost point of the continental U.S. in Key West, the sun reaches an angle of about \(89^{\circ}\) at solar noon on the Summer Solstice. This single degree difference ensures a small shadow is still cast, even at the peak of summer.
As one moves farther north in the continental U.S., the sun’s maximum angle decreases proportionally. For example, in New York, situated near \(40^{\circ}\) North latitude, the sun’s highest angle on the summer solstice is approximately \(73.5^{\circ}\). This variation explains why solar intensity and shadow lengths differ significantly across the country.