When is the primary day of fall in 2024?
A fastidiously worded reply is that on Sunday, Sept. 22, at 8:44 a.m. Jap daylight time (5:44 a.m. Pacific daylight time) autumn begins astronomically within the Northern Hemisphere, and spring within the Southern. At that second, the solar can be shining immediately overhead as seen from some extent within the equatorial Atlantic Ocean, 461 miles (743 km) south-southwest of Monrovia, Liberia.
This date (like final March 20) known as an equinox, from the Latin for “equal night time,” alluding to the truth that day and night time are then of equal size worldwide. However this isn’t essentially so.
Not so Equal
The definition of the equinox as being a time of equal day and night time is a handy oversimplification. For one factor, it treats night time as merely the time the solar is beneath the horizon, and fully ignores twilight. If the solar had been nothing greater than some extent of sunshine within the sky and if the Earth lacked an environment, then on the time of an equinox the solar would certainly spend one half of its path above the horizon and one half beneath. However in actuality, atmospheric refraction raises the solar’s disk by greater than its personal obvious diameter whereas it’s rising or setting. Thus, once we see the solar as a reddish-orange ball simply sitting on the horizon, we’re an optical phantasm. It’s truly fully beneath the horizon.
Along with refraction hastening dawn and delaying sundown, there may be one other issue that makes daylight longer than night time at an equinox: dawn and sundown are outlined because the occasions when the primary or final speck of the solar’s higher limb is seen above the horizon — not the middle of the disk.
And for this reason if you happen to examine your newspaper’s almanac or climate web page on Wednesday and lookup the occasions of native dawn and sundown, you may discover that the length of daylight, or the period of time from dawn to sundown, nonetheless lasts a bit greater than 12 hours, and never precisely 12 because the time period “equinox” suggests.
In Indianapolis, for example, dawn is at 7:32 a.m. and sundown comes at 7:40 p.m. So, the quantity of daylight isn’t 12 hours, however reasonably 12 hours and eight minutes. Not till Sept. 25, are days and nights actually equal (dawn is at 7:35 a.m., sundown coming 12 hours later).
And on the North Pole, the solar at present is tracing out a 360-degree circle across the complete sky, showing to skim simply above the sting of the horizon. For the time being of this 12 months’s Autumnal Equinox, it ought to theoretically disappear fully from view, and but its disk will nonetheless be hovering simply above the horizon. Not till practically 51 hours later will the final speck of the solar’s higher limb lastly drop fully out of sight.
This sturdy refraction impact additionally causes the solar’s disk to look oval when it’s close to the horizon. The quantity of refraction will increase so quickly because the solar approaches the horizon, that its decrease limb is lifted greater than the higher, distorting the solar’s disk noticeably.
Not as darkish because it appears
Sure astronomical myths die onerous. One in every of these is that your complete arctic area experiences six months of daylight and 6 months of darkness. Usually, “night time” is just thought-about to be when the solar is beneath the horizon, as if twilight did not exist. This fallacy is repeated in innumerable geography textbooks, in addition to journey articles and guides. However twilight illuminates the sky to some extent each time the solar’s higher rim is lower than 18-degrees beneath the horizon. This marks the restrict of astronomical twilight, when the sky is certainly completely darkish from horizon to horizon.
There are two different kinds of twilight. Civil (brilliant) twilight exists when the solar is lower than 6-degrees beneath the horizon. It’s loosely outlined as when most outside daytime actions might be continued. Some day by day newspapers present a time when you must flip in your automotive’s headlights. That point often corresponds to the top of civil twilight.
So even on the North Pole, whereas the solar disappears from view for six months starting on Sept. 24, to state that “complete darkness” instantly units in is hardly the case! Civil twilight doesn’t finish there till Oct. 8.
When the solar drops all the way down to 12-degrees beneath the horizon it marks the top of nautical twilight, when a sea horizon turns into troublesome to discern. In truth, on the finish of nautical twilight most individuals will regard night time as having begun. On the North Pole we’ve got to attend till Oct. 24 for nautical twilight to finish. Lastly, astronomical twilight — when the sky certainly turns into fully darkish — ends on Nov. 13. It then stays perpetually darkish till Jan. 28 when the twilight cycles start anew. So, on the North Pole the length of 24-hour darkness lasts virtually 11-weeks, not six months.
Joe Rao serves as an teacher and visitor lecturer at New York’s Hayden Planetarium. He writes about astronomy for Pure Historical past journal, the Farmers’ Almanac and different publications.