12/09/2022 – Ephemeris – The earliest sunset of the year

This is Bob Moler with Ephemeris for Friday, December 9th. Today the Sun will be up for 8 hours and 53 minutes, setting at 5:02, and it will rise tomorrow at 8:09. The Moon, 2 days past full, will rise at 5:59 this evening.

We are at the period of time of the earliest sunset, in the middle of a 13-day stretch where the Sun sets within the same minute. We are still 12 days from the winter solstice, the day of the shortest daylight hours, on the 21st. The reason is twofold. The Sun is near its farthest position south of the equator, where the longitude lines are closer together, so it takes less time to cross them. 15 degrees in longitude equals one hour in Earth’s rotation. Add to that we are less than a month from Earth’s perihelion in its orbit of the Sun, that is at its closest, and is moving faster than average. The combined effects delay sunrise and sunset, from what they’d be if the Sun was on the equator and the Earth’s orbit was circular. We will have our latest sunrise on January 2nd.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT –5 hours). They may be different for your location.

Addendum

How the Sun’s declination affects how rapidly it appears to cross time lines (meridians)

Table of Earliest and Latest Sunrises and Sunsets during the year for Interlochen/Traverse City area of Michigan.

In December the Earth is approaching perihelion, its closest to the Sun, so it moves faster than average. This makes the Sun to appear to move faster eastward against the stars in our sky. This tends to make our sunrises, apparent local noons and sunsets later than they would otherwise be. This makes sunset bottom out early and extends the date of the latest sunrise

For the June or summer solstice around here, the Earth is near aphelion, its farthest from the Sun where the Earth is at its slowest in its orbit. By reflection, the Sun appears to move its slowest against the stars in our sky. This effect works against the high latitude effect, making the effect smaller. Looking at the table above, the days between the earliest and latest times is shorter for the summer solstice than for the winter solstice.

12/09/2019 – Ephemeris – Tomorrow is the earliest sunset

Ephemeris for Monday, December 9th. Today the Sun will be up for 8 hours and 54 minutes, setting at 5:02, and it will rise tomorrow at 8:08. The Moon, 3 days before full, will set at 6:15 tomorrow morning.

In twelve days we will have the shortest day in terms of daylight hours. But the change isn’t uniformly distributed in the morning and evening. Tonight we will have the earliest sunset. Sunset times have been within the same minute for the last few days and will continue for the next few. The latest sunrise will occur on January second. The reason is that the Sun is traveling faster eastward than average because the Earth is closer to it now, so the Earth’s rotation takes a little longer each day to catch up with it. Near the solstice the Sun is at a higher latitude, where the longitude lines are closer together so it crosses them faster. A smaller effect exists in June because the Sun is farther away and its slowness subtracts from the higher latitude effects.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

This figure 8 is called an analemma. One can find it on old globes in the Pacific Ocean. Explanation below. Created using my LookingUp program.

An Analemma graphically demonstrating the actual Sun’s relation to the mean Sun during the year.

From my December 2nd 2014 post with updates:

The analemma is a graphical representation of a daily value called the Equation of Time. It’s best known use is in corrections to sundial time. The vertical axis is the sun’s declination or north-south position. It is highest at summer solstice and lowest at winter solstice. It is the result of two effects: the tilt of the Earth’s axis to the plane of the Earth’s orbit around the Sun, and the change in the Earth’s velocity around the Sun as the Earth moves from perihelion, its closest to the Sun in early January to aphelion, its most distant in July.

If the Earth’s orbit were circular, and it orbited the Sun at the same speed. The analemma would be skinnier and the north and south lobes would be of equal size. Since we’re closer to the Sun in the winter, we move faster than average around the Sun, so it appears to move faster eastward. That combines with the faster appearing movement of the sun crossing the closer hour lines at higher and lower declinations (latitudes). In the diagram above note that the vertical hour lines are slightly closer together at the bottom and the top, so the Sun, moving eastward each day crosses them quicker. Near the winter solstice the two effects work together making sunrise and sunset trending to be later than normal. For the summer solstice the eastward speed of the sun is slower than normal, because we’re farther from the Sun. This works against the effect of the earth’s tilt but cannot completely negate it, making the top of the loop smaller than the one at the bottom. The arrows show the speed and direction of the Sun at the solstices.

In my month preview posts I generally mention the time of local noon for mid month.  I didn’t do it for this month. If the equation of time is zero our local solar noon, in the Interlochen/Traverse City area, the Sun is due south at 12:43 p.m. during Eastern Standard Time, because that time meridian of 75 degrees longitude runs through Philadelphia PA.  During daylight time it’s 60 degrees longitude, which clips the eastern edge of Nova Scotia.  On the top figure of the analemma, if the Sun is west of center, the Sun is fast and to the east is slow.  It changes rapidly in December, at the bottom end of the figure 8 when the Sun is farthest south.  Local solar noon on the first is at 12:31 p.m., and it will slow to 12:45 p.m. on the 31st.

To see real analemmas in the sky search for analemma images on the Internet. It takes a year to photograph one.

 

12/08/2017 – Ephemeris – The earliest sunset of the year is tomorrow night

Ephemeris for Friday, December 8th. The Sun will rise at 8:06. It’ll be up for 8 hours and 55 minutes, setting at 5:02. The Moon, 2 days before last quarter, will rise at 11:04 this evening.

In thirteen days we will have the shortest day in terms of daylight hours. But the change isn’t uniformly distributed in the morning and evening. Tomorrow evening we will have the earliest sunset. Sunset times have been within the same minute for the last few days and will continue for the next few. The latest sunrise will occur on January second. The reason is that the Sun is traveling faster eastward than average, so the Earth’s rotation takes a little longer each day to catch up with it. Near the solstice the Sun is at a higher latitude, where the longitude lines are closer together, also the Earth is nearing its closest to the Sun, so moves faster its orbit adding to the effect. The effect exists in June but isn’t as noticeable.

The times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.

Addendum

I had more complete thoughts about earliest and latest sunrises and sunsets earlier this year: https://bobmoler.wordpress.com/2017/03/13/03132017-ephemeris-more-thoughts-about-yesterdays-time-change/. Check the addendum.