Summer Solstice 2019: When Is the First Day of Summer and Why Is It the Longest Day of the Year?

We are fast approaching the 2019 summer solstice in the northern hemisphere—the point in the year at which the Earth's north pole is tilted furthest toward the sun.

Technically, the solstice takes place at a specific point in time, which this year will be 11:54 a.m. EDT on Friday, June 21, according to the Old Farmer's Almanac. But usually when people talk about the solstice, they are referring to the entire day on which it occurs.

This day is the longest of the year in the Northern Hemisphere—meaning it has the most hours of sunlight and the shortest night—because our star will appear at its highest in the northern sky, directly over the Tropic of Cancer. Thus it will take longer to rise and set.

In contrast, those in the southern hemisphere will experience the shortest day of the year with the least sunlight hours and the longest night.

The reason we have a solstice at all is because the Earth is tilted on its axis by 23.5 degrees relative to the plane of its orbit. As a consequence, each hemisphere experiences half a year tilted toward the sun and the other half tilted away from it.

The differences in the tilt mean that both hemispheres are struck by varying levels of radiation from the sun over the course of the year, giving rise to the seasons.

So when the northern hemisphere is tilted toward the sun—as is the case now—it experiences summer because radiation from the star is striking it at a more direct angle, thus creating warmer temperatures.

As you move closer to the equator, this effect becomes less noticeable. This is why the equatorial region does not truly experience summer and winter: because the radiation striking it from the sun is more constant.

In astrological terms, the June solstice marks the end of spring and start of summer for the northern hemisphere—which will end with the fall equinox on September 23.

However, when speaking in meteorological terms, summer actually begun on June 1 and will end on August 31. The meteorological year is divided up slightly differently to the astrological year, taking into account the months of the calendar and annual temperature cycles.

The summer solstice does not always fall on the same date. In fact it can occur anywhere between June 20 and 22. This is due to slight differences in the Gregorian calendar—the most widely used calendar system in the world—which has 365 days and the "solar year," essentially, the time it takes for the Earth to complete one revolution around the sun (365.242199 days.)

To ensure that the calendar is synchronized with the seasons, we add an extra day to the year around every four years—what's known as a leap year.

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Stock photo: During the June Solstice the sun will appear at its highest in the northern sky. iStock