Twilight and the myths of the equinox and 6-thirty day period polar evening
Wednesday (Sept. 22) marks the equinox, which, thanks to its Latin title which means “equal evening,” is usually considered of as the day when dim and gentle every single claim 12 several hours.
But that is not the situation, and twilight is to blame for the confusion surrounding the astronomy of an equinox.
Let us backtrack. Astronomically talking, tumble commences in the Northern Hemisphere (and spring in the south) on Sept. 22 at 3:21 p.m. EDT (1721 GMT. That’s when the sunshine will be shining specifically overhead as found from a position in the equatorial Pacific Ocean, 1,600 miles (2,580 kilometers) southwest of Mexico Metropolis.
But Northern Hemisphere locations will nevertheless see marginally a lot more than 12 hrs of daylight, in spite of the equinox designation.
Linked: The entire Harvest Moon of 2021 rises tonight: This is what to search for
Not so equal
The definition of the equinox as remaining a day of equal daylight and night time is a handy oversimplification.
For just one point, it treats evening as only the time the sunshine is beneath the horizon, absolutely disregarding twilight. If the solar were nothing a lot more than a stage of gentle in the sky and if the Earth lacked an environment, then at the time of an equinox the sun would without a doubt devote one particular 50 % of its path earlier mentioned the horizon and one particular fifty percent down below. But in actuality, atmospheric refraction raises the sun’s disk by extra than its have obvious diameter when it is increasing or location. Thus, when we see the sunshine as a reddish-orange ball just sitting down on the horizon, we’re searching at an optical illusion — the sunlight is essentially fully under the horizon.
Furthermore, dawn and sunset are described as the situations when the initial or very last speck of the sun’s higher limb is visible over the horizon — not the middle of the disk. This is why if you search up the situations of nearby sunrise and sunset on Wednesday, you’ll notice that the length of daylight, or the total of time from dawn to sunset, still lasts a bit additional than 12 hours.
In Chicago, for instance, dawn is at 6:38 a.m. and sunset will come at 6:47 p.m. So, the sum of daylight is not 12 hours, but relatively 12 hrs and 9 minutes. Not until Saturday (Sept. 25) are the working day and night actually equal (dawn is at 6:41 a.m., sunset comes 12 hours later).
On Sept. 22 at the North Pole, the solar traces out a 360-degree circle about the full sky, appearing to skim just earlier mentioned the edge of the horizon. At the minute of this year’s autumnal equinox, the sun need to theoretically disappear wholly from watch, and but its disk will still be hovering just earlier mentioned the horizon. Not till 50 hrs and 44 minutes afterwards will the previous speck of the sun’s higher limb finally fall wholly out of sight, a lot more than two days later.
This solid refraction influence also results in the sun’s disk to appear oval when it is near the horizon. The amount of refraction increases so fast as the sunlight approaches the horizon, that its reduced limb is lifted much more than the upper, distorting the sun’s disk significantly.
Not as darkish as it seems
Sure astronomical myths die hard. A person of these is that that the whole arctic area activities 6 months of daylight and six months of darkness. Usually, “night” is merely deemed to be when the sunlight is beneath the horizon, as if twilight failed to exist. This fallacy is repeated in innumerable geography textbooks, as very well as travel content articles and guides.
But twilight illuminates the sky to some extent every time the sun’s higher rim is much less than 18 levels below the horizon. This marks the limit of astronomical twilight, when the sky is in fact totally dim from horizon to horizon.
There are two other kinds of twilight. Civil (dazzling) twilight occurs when the sunlight is a lot less than 6 degrees beneath the horizon and is loosely outlined as when most outside daytime functions can be continued. (Some each day newspapers supply a time when you should change on your car’s headlights, commonly corresponding to the stop of civil twilight.)
So even at the North Pole, when the sun disappears from see for six months commencing on Sept. 24, to state that “full darkness” promptly sets in is rarely the scenario! Civil twilight does not finish there until Oct. 8.
The previous type of twilight is nautical twilight, which finishes when a sea horizon turns into hard to discern, generally when the solar drops down to 12 levels under the horizon. At the conclusion of nautical twilight, most folks will regard night as possessing started. At the North Pole we have to wait until finally Oct. 24 for nautical twilight to close.
Eventually, astronomical twilight — when the sky in fact gets completely dim — ends on Nov. 13. It then stays perpetually dim right up until Jan. 28, when the twilight cycles commence anew. So, at the North Pole the period of 24-hour darkness lasts practically 11 months, not six months.
Joe Rao serves as an teacher and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Purely natural Historical past journal, the Farmers’ Almanac and other publications. Adhere to us on Twitter @Spacedotcom and on Fb.
Wednesday (Sept. 22) marks the equinox, which, thanks to its Latin title which means “equal evening,” is usually considered of as the day when dim and gentle every single claim 12 several hours.
But that is not the situation, and twilight is to blame for the confusion surrounding the astronomy of an equinox.
Let us backtrack. Astronomically talking, tumble commences in the Northern Hemisphere (and spring in the south) on Sept. 22 at 3:21 p.m. EDT (1721 GMT. That’s when the sunshine will be shining specifically overhead as found from a position in the equatorial Pacific Ocean, 1,600 miles (2,580 kilometers) southwest of Mexico Metropolis.
But Northern Hemisphere locations will nevertheless see marginally a lot more than 12 hrs of daylight, in spite of the equinox designation.
Linked: The entire Harvest Moon of 2021 rises tonight: This is what to search for
Not so equal
The definition of the equinox as remaining a day of equal daylight and night time is a handy oversimplification.
For just one point, it treats evening as only the time the sunshine is beneath the horizon, absolutely disregarding twilight. If the solar were nothing a lot more than a stage of gentle in the sky and if the Earth lacked an environment, then at the time of an equinox the sun would without a doubt devote one particular 50 % of its path earlier mentioned the horizon and one particular fifty percent down below. But in actuality, atmospheric refraction raises the sun’s disk by extra than its have obvious diameter when it is increasing or location. Thus, when we see the sunshine as a reddish-orange ball just sitting down on the horizon, we’re searching at an optical illusion — the sunlight is essentially fully under the horizon.
Furthermore, dawn and sunset are described as the situations when the initial or very last speck of the sun’s higher limb is visible over the horizon — not the middle of the disk. This is why if you search up the situations of nearby sunrise and sunset on Wednesday, you’ll notice that the length of daylight, or the total of time from dawn to sunset, still lasts a bit additional than 12 hours.
In Chicago, for instance, dawn is at 6:38 a.m. and sunset will come at 6:47 p.m. So, the sum of daylight is not 12 hours, but relatively 12 hrs and 9 minutes. Not until Saturday (Sept. 25) are the working day and night actually equal (dawn is at 6:41 a.m., sunset comes 12 hours later).
On Sept. 22 at the North Pole, the solar traces out a 360-degree circle about the full sky, appearing to skim just earlier mentioned the edge of the horizon. At the minute of this year’s autumnal equinox, the sun need to theoretically disappear wholly from watch, and but its disk will still be hovering just earlier mentioned the horizon. Not till 50 hrs and 44 minutes afterwards will the previous speck of the sun’s higher limb finally fall wholly out of sight, a lot more than two days later.
This solid refraction influence also results in the sun’s disk to appear oval when it is near the horizon. The amount of refraction increases so fast as the sunlight approaches the horizon, that its reduced limb is lifted much more than the upper, distorting the sun’s disk significantly.
Not as darkish as it seems
Sure astronomical myths die hard. A person of these is that that the whole arctic area activities 6 months of daylight and six months of darkness. Usually, “night” is merely deemed to be when the sunlight is beneath the horizon, as if twilight failed to exist. This fallacy is repeated in innumerable geography textbooks, as very well as travel content articles and guides.
But twilight illuminates the sky to some extent every time the sun’s higher rim is much less than 18 levels below the horizon. This marks the limit of astronomical twilight, when the sky is in fact totally dim from horizon to horizon.
There are two other kinds of twilight. Civil (dazzling) twilight occurs when the sunlight is a lot less than 6 degrees beneath the horizon and is loosely outlined as when most outside daytime functions can be continued. (Some each day newspapers supply a time when you should change on your car’s headlights, commonly corresponding to the stop of civil twilight.)
So even at the North Pole, when the sun disappears from see for six months commencing on Sept. 24, to state that “full darkness” promptly sets in is rarely the scenario! Civil twilight does not finish there until Oct. 8.
The previous type of twilight is nautical twilight, which finishes when a sea horizon turns into hard to discern, generally when the solar drops down to 12 levels under the horizon. At the conclusion of nautical twilight, most folks will regard night as possessing started. At the North Pole we have to wait until finally Oct. 24 for nautical twilight to close.
Eventually, astronomical twilight — when the sky in fact gets completely dim — ends on Nov. 13. It then stays perpetually dim right up until Jan. 28, when the twilight cycles commence anew. So, at the North Pole the period of 24-hour darkness lasts practically 11 months, not six months.
Joe Rao serves as an teacher and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Purely natural Historical past journal, the Farmers’ Almanac and other publications. Adhere to us on Twitter @Spacedotcom and on Fb.