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Friday's start to summer marks longest day of year

But season starts (marginally) earlier next year

JACKSONVILLE, Fla – Summer starts Friday but planning ahead could lead to confusing changes.

If you are ready to set your summer party for the longest day of the year and start of summer, make sure its the right day.

Astronomers and scientists use the date of the June solstice to mark the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.

Even though most people consider June 21 as the date of the June solstice, it can happen anytime between June 20 and 22. This year it will be on June 21 but switches to June 20 next summer. 

Nobody needs to worry about planning for a June 22 solstice anytime soon. They are so rare the last June 22 solstice took place in 1975 and there won't be another one until 2203.

The date of the Summer solstice can change from year to year because almost every four years a leap year is added to the Gregorian Calendar.

The Earth takes a bit more than 365 days to make a complete trip around the Sun. 

To be exact, it takes the Earth about 365.25 days on average to make one revolution around the Sun.

This means that the start to Summer occurs about 6 hours later than the previous year's June Solstice.

For instance, the 2018 solstice occurred on June 21 at 6:07 AM EDT. This year the 2019 solstice takes place at 11:54 AM EDT Friday,  about six hours later than in the previous year. 

Next year, 2020 is a leap year and summer will start a day earlier on June 20 at 5:45 p.m.


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