Relative day lengths

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You can see that on March 21, the spring equinox, all the cities listed have approximately 12 hours of sunlight. On June 21, the summer solstice and longest day of the year for the northern hemisphere, the day length ranges from about 13 � hours in Hawaii to 24 hours in northern Alaska. On December 21, the shortest day of the year, the day length ranges from almost 11 hours in Hawaii to 0 hours in Northern Alaska.

This diagram shows the earth’s position relative to the sun on the summer solstice in the northern hemisphere. The summer solstice is the longest day of the year (that is, the day with the most hours of daylight). When it is the summer solstice in the northern hemisphere, it is the winter solstice in the southern hemisphere—the shortest day of the year.

Remember that for the earth:

• one day equals one complete revolution on its axis.
• one year equals one complete orbit around the sun.

On the diagram you’ll see that the earth’s axis of rotation isn’t perpendicular to the sun. Rather, the earth’s axis is tipped at a 23.5 degree angle. This angle remains constant as the earth orbits the sun. When the northern hemisphere is tipped toward the sun, it is summer there. Six months later, when the earth’s orbit reaches the other side of the sun, the northern hemisphere will be tipped away from the sun, and it will be winter.

Find the equator on the diagram. Moving north, we’ve labeled the latitudes of Honolulu, Hawaii, Burlington, Vermont, and Barrow, Alaska. South of the equator, in the southern hemisphere, you’ll find Hamilton, Australia. Following are these cities’ latitudes, from northermost to southermost:

• Barrow, Alaska: 71 degrees North
• Burlington, Vermont: 44 degrees North
• Honolulu, Hawaii: 21 degrees North
• Equator: 0 degrees
• Hamilton, Australia: 38 degrees South

Note the shading on the "dark side of the earth"—the half of the planet that is not receiving sunlight.

Now let’s put all these factors together to show the difference in relative day lengths of each of these cities. The dark lines indicate the portion of the day that is in daylight. The dotted/dashed line indicates the portion that is in darkness.

Let’s start with Alaska. You can see from the diagram that at no time of the earth’s revolution is Alaska out of the daylight. On this day, the sun never sets—the "day length" is 24 hours!

Moving south, you can see that most of Vermont’s day is lit by sunlight; only a small percentage of the day, maybe one third, is in darkness. On the summer solstice, Vermont’s daylength is 15.5 hours.

In Hawaii, slightly more than half the revolution is in daylight, or 13.5 hours.

In Australia, less than half the revolution receives daylight, only 9 hours.

The purpose of all this is to show that day length, or number of hours of daylight, varies dramatically with latitude at certain times of the year. The further you get from the equator, the more dramatic the difference in the number of hours of daylight during the longest and shortest days of the year.

Now let’s look at another factor besides day length that influences the onset of flowering.