Most of us really appreciate the increased period of daylight that comes when winter moves toward spring and spring moves toward summer. From late December until late June, nearly everywhere in the Northern Hemisphere enjoys later sunsets and warmer weather.

In mid spring, the Earth's highest temperatures are typically found in the desert areas of northern Africa and the Middle East as well as in parts of Pakistan and India, which have not yet made the transition to the wet monsoon. Temperature in these areas often soar above 110 degrees. In contrast, the Earth's coldest temperatures are almost always found on the high plateaus of eastern Antarctica. Today, for instance, the temperature at the Russian research station of Vostok is -87 degrees F! The lowest temperatures in the Northern Hemisphere occur in Siberia, northern Canada, Alaska, or Greenland. Even in late April and early May, minimum temperatures below zero degrees F are normally experienced somewhere in the Arctic. In the past few days, for the first time since October, the Northern Hemisphere minimum temperature have failed to drop below zero degrees F (as recorded at official weather stations). For example, on May 19, the world's lowest temperature (outside of Antarctic) was 0 degrees F and was recorded at Saskylakh, Russia (at 72 degrees north latitude [N] in central Siberia, just south of the Arctic Ocean).

In late December, the Sun never ventures above the horizon in Saskylakh, but now the Sun is up all day! In New Orleans, which has a latitude of 30 degrees N, the Sun now remains above the horizon for about 13 hours and 45 minutes, and by June 21, another 15 minutes of sunlight will be available. At about 40 degrees north latitude, the latitude of Philadelphia, the Sun stays up for approximately 15 hours a day now, and by the time the summer solstice arrives, another 30 minutes or so of sweet sunshine will be garnered. At a latitude of 50 degrees N, in Winnipeg, Canada for example, the Sun shines for nearly 15 hours and 45 now, and in another month, the daylight will increase to about 17 1/2 hours.

Ok, it's no big surprise that the amount of daylight increases more noticeably as one goes from south to north in late spring. However, the by mid May, the Sun is above the horizon 24-7 at all latitudes north of 70 degrees N, and thus these areas can't gain any more sunlight as spring progresses. Outposts like Thule, Greenland, Alert and Eureka, Canada, and fishing villages like Biruli and Tempa, Russia, are all now bathed in constant sunlight, or they would be if it wasn't so cloudy on the tundra during the spring and summer. Even though the Sun is up all day in these places, the air above the still very cold waters of the Arctic Ocean, when it moves inland over the warming tundra, results in persistent spells of fog and cloudiness.

The largest town farthest north on the globe is Longyearbyaen, Norway (on the island of Spitzbergen). At 77 degrees N, the Sun won't set until early September. Of course, once it does, it's payback time - from late September until late March, it's dark and cold. "What a long year it has been" must surely be the original name of Longyearbyen.

It's kind of depressing to think that in just 6 weeks, we'll start to lose a little of the daylight we cherished so during the winter. But in one area of the Northern Hemisphere, the Sun is already setting a bit sooner now than last month. How's that possible?

At noon now, the Sun is directly overhead a latitude of approximately 15 degrees N. Areas to the north of this see increasing daylight through June 21 or so. Conversely, areas in the Southern Hemisphere are experiencing the opposite - waning daylight. However, areas between 15 degrees N and the Equator are caught in a kind of "no man's land." According to the Astronomical Almanac, at the Equator on the Prime Meridian (0 degrees longitude), the Sun is now up for about 12 hours and 7 minutes, while at 10 degrees N, it's up for 12 hours and 37 minutes. A month from now at the Equator, the Sun will be up for a full minute more, but at 10 degrees N, it'll be up 5 minutes longer.

Strangely, until mid September, the month May is the time of the earliest sunset at the Equator and for latitudes about a degree or so north of the Equator. The Sun dips below the horizon about 6:00 p.m. now on the Equator and rises about 5:53 a.m (at 0 degrees longitude). It's setting a couple of minutes earlier now than it did a month ago and will a month from now. However, it's rising earlier now than anytime until mid September. So, actually the days are still getting longer even at the Equator. In the mid latitudes, when the Sun moves away from us (it's noontime position in the sky is further south), the days get shorter, but this isn't always the case near the Equator. Think of it this way, even though the Sun is moving away from the Equator now, and will do so over the next few weeks, the Northern Hemisphere of the Earth is continuing to tilt toward the Sun, thus resulting in longer and longer daylight hours for people living northward of the Equator. On about June 21, the Sun at noon will be directly overhead on the Tropic of Cancer (23 1/2 degrees N). After this time, the Sun's position in the sky will gradually shift southward and the length of daylight will gradually decrease, except near the Equator.

There are two main effects that determine sunrise and sunset: the first is the declination of the Sun; the second is the non-circular orbit of the Earth. The second is a much smaller effect than the first. In regards to declination, if the Earth's orbit were a perfect circle, then regardless of where one was in the Northern Hemisphere, through winter and 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; the change in length of day near the pole is large.

Of course, the Earth's orbit is not a perfect circle. So the Earth's speed is not a constant. When the Earth is closer to the Sun, and thus moving a bit faster than average, both sunrise and sunrise shift together to occur a bit later than they would if the motion were purely circular. When the Earth is further from the Sun, and thus moving a bit slower than average, both sunrise and sunrise shift together to occur a bit earlier than if the motion were purely circular. We follow these changes with the "equation of time", or "analema".

The effect is not a large one, so at mid to high latitudes, we don't notice any shift in sunrise/set times - we just notice the increasing daylight. However, because the change in length of day near the Equator is small (it's still increasing, just not by much) the effect of the non circular orbit is noticeable there. The Earth is moving a bit slower than average from February through May, so both the sunrise and sunset shift occur earlier and earlier.

The bottom line is that all areas in the Northern Hemisphere are enjoying longer days, but near the equator, the sun is setting a bit earlier each day in May and a bit later at the higher latitudes.

Because not all of the Sun, or the moon for that matter, is seen to rise or set at once, a strict definition of sunrise and sunset is used by the US Naval Observatory in Washington D.C., for purposes of calculating sunrise and set and the length of day. "Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the Sun is on the horizon, considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface."

Remember that the further you are from where the Sun is overhead at noon, the more the path of the Sun is slanted at sunrise and sunset. Therefore, 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 a minute or two for the Sun to clear the horizon, there's a few minutes more than 12 hours of daylight at 0 degrees latitude each and every day of the year - there's never less than 12 hours of daylight on the Equator.

For more about this see the Naval Observatory web page http://www.usno.navy.mil/ 

Well, this is was the final question for the current year. We'll pick up with the questions again in early September. Hope you have a good summer!