What’s midday to you?
When is it midday for you? That’s not as simple a query to reply as you would possibly assume! What do you imply by midday? Do you outline it by your clock or smartphone? Or the gnawing in your abdomen that claims it’s time for lunch? Effectively, you would possibly wish to assume once more!
We within the Northern Hemisphere usually say the primary day of summer season comes on the June solstice, which, in 2023, falls on June 21. However – it doesn’t matter what day it’s – when midday happens relies upon partially in your location and partially in your definition of midday. On the day of a June solstice, noontime shadows are only a hair shorter than the day earlier than. That’s as a result of, on the June solstice, Earth’s Northern Hemisphere is tilted most sunward for the yr, and the sun rises highest in our Northern Hemisphere sky, yielding the yr’s shortest noon shadows.
Discover that I mentioned noon, slightly than simply midday. Often when most of us say midday, we imply 12 p.m. on the clock. However that might not be what astronomers imply by midday.
Shadows at midday are shortest across the June solstice
Across the time of the June solstice, shadows are shortest when the sun is due south within the sky. We are saying that the sun is crossing the meridian. However the sun’s meridian crossing not often happens at precisely straight-up midday, in keeping with the clock. The time at which the sun crosses the meridian was referred to as excessive midday as a result of that’s when the sun is highest within the sky. In the present day we astronomers typically consult with it as transit time or native solar midday.
You’ll be able to carry out an experiment your self
In truth, you’ll be able to carry out an experiment that simulates an commentary by Eratosthenes greater than 2,000 years in the past, by means of which he obtained the primary correct measurement of the dimensions of the Earth.
You’ll be able to assemble a easy system referred to as a gnomon (pronounced NO-mun) or shadow keep on with calculate how excessive the sun is, with easy trigonometry. Simply measure the peak of the gnomon (H) and the size of the shadow (L) at native solar midday. Then the angular top of the sun is the arctangent of H/L. For accuracy, the gnomon have to be very straight, the bottom degree, and the measurements exact. For instance, in Denver, it is going to be about 73.75 levels excessive. How excessive is it the place you reside?
However earlier than you can also make the measurement, you could know when native solar midday happens the place you’re situated. Earlier than the invention of the telegraph within the nineteenth century, each locality outlined midday by the point when the sun crossed the meridian. Due to geographical location, when the sun crosses the meridian as seen in Denver, in Grand Junction (about 180 miles to the west) the sun hasn’t reached the meridian but, and gained’t for one more 14 minutes or so. It takes about 14 minutes for the Earth to show far sufficient to carry the sun to the meridian in Grand Junction after it passes the meridian in Denver.
Why we’ve time zones
At a time when the quickest type of communication was a stagecoach or the Pony Express, this distinction in time didn’t matter. However when near-instantaneous communication turned obtainable with the telegraph, folks steadily realized {that a} standardization of time was mandatory. Thus the idea of time zones was developed in 1884, however not adopted formally within the U.S. till 1918.
However even contemplating time zones, the precise time of transit varies by means of the yr due to the Earth’s various pace in its orbit across the sun. At occasions the planet has to show a bit extra from one transit to the following, and at occasions it wants to show a bit much less. It is because the change in Earth’s pace (attributable to various distance to the sun by means of the yr) causes the sun’s obvious movement throughout the sky to vary. There may be additionally a variation as a result of tilt of the Earth that impacts the sun’s obvious movement within the sky. That implies that the actual sun (which crosses the meridian at native solar midday) is as a lot as 16 minutes sooner or slower than the regular however fictitious imply sun that defines clock-time midday for the varied time zones.
And don’t neglect daylight saving time
After which after all there may be the complication of “daylight saving time,” which actually saves nothing however merely offsets the clocks by an hour.
Now you can also make all of the calculations to determine transit time or native solar midday for your self, however you don’t must go to the difficulty. Simply go to the Sunrise Sunset Calendar web site and have it calculate it for you.
Listed below are some examples of typical transit occasions, or native solar noons, for June 21, as calculated by the Dawn Sundown Calendar web site:
| Jap Time Zone | Central Time Zone | Mountain Time Zone | Pacific Time Zone | ||||
| New York | 12:58 p.m | Chicago | 12:52 p.m. | Denver | 1:02 p.m. | Los Angeles | 12:55 p.m. |
| Miami | 1:22 p.m. | Little Rock | 1:11 p.m. | Albuquerque | 1:08 p.m. | Seattle | 1:11 p.m. |
| Detroit | 1:34 p.m. | Kansas Metropolis (KS) | 1:21 p.m. | Salt Lake Metropolis | 1:29 p.m. | San Francisco | 1:11 p.m. |
| Atlanta | 1:39 p.m. | Houston | 1:23 p.m. | Phoenix | 12:30 p.m. * | Portland | 1:12 p.m. |
* Be aware that Phoenix is on Normal, not Daylight, Time. Additionally be aware that the precise second of the summer season solstice isn’t associated to the time of native solar midday.
Oh, and by the best way, for those who’re out within the sun – considering the passage of the sun and the time of midday for you – don’t neglect your sunscreen.
Backside line: What’s the definition of midday? An exploration of how astronomers take into consideration noontime.
Read more: Equation of time: Why times vary for your high noon by Guy Ottwell
