Do the dates for the latest sunrise and earliest sunset during the winter occur at nearly the same time for all latitudes?

While in the U.S., January has been a rather mild month, in northern and central Europe, it has been snowy and downright cold. In the depths of winter, most of us cheerfully anticipate the increased period of daylight that comes when we move toward spring. In just 4 months, outposts like Thule, Greenland and Eureka, Canada, while now gripped in cold and darkness, will be bathed in constant sunlight. Even though the Northern Hemisphere of the Earth is still tilting away from the Sun, which is now positioned south of the Equator at noon, the Sun is gradually "moving" northward, and will do so over the next 20 weeks or so. Note that it’s the Earth that’s actually doing the moving, not the Sun.

In the Northern Hemisphere, the winter solstice occurs when the mid day position of the Sun is over the Tropic of Capricorn (23 1/2 degrees south latitude). This is the day with the shortest daylight period, and it usually falls on December 21 or 22. However, for folks in Philadelphia, Pennsylvania, Ankara, Turkey and Beijing, China (40 degrees north latitude), the date of the earliest sunset occurs in early December (between about December 4 and December 12) and the date of the latest sunrise is in early January (between January 2 and January 10).

Further north, though, these dates are a bit less skewed. For instance, in Moscow, Russia (56 degrees north latitude), the date of the earliest sunset is from December 12 to December 18, and the latest sunrise is between about December 28 and December 30. As you move further south, for example near, Mexico City, Mexico or Bombay, India (19 degrees north latitude), the earliest sunset occurs between about November 22 to November 30, whereas the latest sunrise is between January 14 and January 22. At 5 degrees north latitude, the dates of the earliest sunset and latest sunrise are from about November 2 to November 6 and February 3 to February 7, respectively.

What's going on here? There are two main effects that determine the times of sunrise and sunset: the first is the declination of the Sun (generally the largest effect); the second is the Equation of Time, which is made up of the non-circular orbit of the Earth and the obliquity of the ecliptic. Since your eyes are likely already pretty well glazed over, we’ll just touch upon this briefly.

Declination is essentially the Sun’s latitude in the sky – always between 23½ degrees south and 23½ degrees north latitude. If the Earth's orbit were a perfect circle, then regardless of where one was in the Northern Hemisphere, throughout spring, sunrise would get earlier and sunset later each day. The higher the latitude, the greater the effect. So as the months’ progress, the change in length of day near the Equator is small, while the change in length of day near the North Pole is considerable.

Of course, the Earth's orbit isn’t a perfect circle; it’s elliptical. So if you know anything about Kepler and his laws of planetary motion, you might recall that the Earth's velocity about the Sun isn't a constant. In January, when the Earth is closer to the Sun (the Earth is at perihelion in early January -- nearer to the Sun), and thus moving a bit faster than average, both sunrise and sunset shift together to occur a bit later than they would if our orbital motion were purely circular. In contrast, when the Earth is moving a bit slower than average (June and July), both sunrise and sunrise shift together to occur a bit earlier than if the motion were purely circular.
Note that at the Equator, because the change in the amount of daylight during the course of the year is pretty small, the effect of our non circular orbit is much more evident there.

Aside from the Earth's tilt and orbital characteristics, how the Sun sets and rises also plays a role in the length of daylight hours. It takes longer for the top of the Sun to fully sink below the horizon in the mid latitudes than it does in the tropics, where it sinks more abruptly. However, because even on the Equator it takes 2 minutes for the Sun to clear the horizon, there's more than 12 hours of daylight there each and every day of the year.

We can't tell for sure whether or not the remainder of the winter will be frigid or tepid, but regardless of the weather, the daylight period will continue to increase.

For more about this see this Naval Observatory site
http://aa.usno.navy.mil/faq/docs/dark_days.html
See also the Earth Science Picture of the Day for January 4, 2006 http://epod.usra.edu/archive/epodviewer.php3?oid=283072 and
the Earth Science Picture of the Day for January 23, 2003
http://epod.usra.edu/archive/epodviewer.php3?oid=123104
Additionally, for more details see Variation in Time of Sunrise
http://www.physics.rutgers.edu/~twatts/sunrise/sunrise.html

This week's question comes from Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Two years ago, Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.