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Autumn begins Thursday, but keep an eye on the tropics

Sailboats at the MIT Sailing Pavilion are ready for Massachusetts Institute of Technology students as part of their Physical Education course.Suzanne Kreiter/Globe Staff

Today is the last full day of astronomical summer. Beginning Thursday evening, fall arrives.

There are all sorts of interesting ways to measure the changing seasons. Solar autumn, the three months of the year when we lose more daylight than at any other time, began a few days into August. The opposite of this would be solar spring, which begins in early February and continues until early May.

Climatological Autumn began Sept. 1, marking the 3 months between climatological summer and climatological winter, which begins Dec. 1. Although this isn’t the season you learned about in grammar school, in a lot of ways it makes the most sense with regard to temperature. By the time we get to December, it really doesn’t feel like autumn anymore.

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September 1st marks the beginning of climatological autumn.NOAA

Astronomical fall, which begins tomorrow night, is simply marking the relationship between the Earth and the Sun. At that moment, the terminator — not Arnold Schwarzenegger’s character but the position of sunrise/sunset — is vertical, north-to-south, from the North Pole to the South Pole. Where it is noon on the equator, the Sun would be directly overhead at that time and there would be no shadow.

The line of night and day is exactly vertical when the autumnal equinox arrives on Thursday just after 9 pm.TimeandDate

Thursday does not mark the time when we have equal day and night. We’ll have to wait until the equilux on Sept. 25, a few days after the equinox.

The equilux occurs on September 25th, a few days after the autumnal equinox.TimeandDate.com

When you get up Friday morning, it’s going to feel like fall with temperatures in the 40s and only reaching the upper 50s or lower 60s in the afternoon. This autumnal weather is not going to last as a warming trend takes us into the weekend. Temperatures will be back near, or above, seasonal normal in the lower 70s Sunday.

It will be quite cool Friday for the first full day of Autumn.NOAA

Finally, although it is autumn, we still have to keep a close eye on the tropics. Hurricane Fiona will make a close pass by Bermuda, but it’s likely to not have a direct impact. An area of concern to the south is likely in the coming days and has the best chance of any storm so far this season of impacting the United States. More on that to come.

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The tropics continue to be a bit active with two named storms Fiona and Gaston and a possible third one becoming Hermine later this week.National Hurricane Center