The Sun stands still

Summer officially arrives in the Northern Hemisphere this week, in the form of the summer solstice, the instant the Sun reaches its most northerly position for those living north of the equator. In the Southern Hemisphere, meanwhile, the Sun reaches its lowest point in their sky, hence this week marks the beginning of southern winter.

On the first day of summer, the Sun is directly overhead at noon for people living at latitude 23.3°N, a region denoted as the Tropic of Cancer. There's nothing cryptic about this value. It's just how much Earth's rotational axis is tilted with respect to the plane of the solar system. If Earth's axis were tilted more, say 30°, the Sun would be overhead at that latitude instead.

As we orbit the Sun, Earth's tilted axis makes the Sun appear to gradually move north to south and back again from any given location in the world. But only in regions within 23.3° of the equator does the Sun actually ever stand directly overhead. If you live at latitude 30° N, the Sun is still 6.7° south of your zenith on the first day of summer (30°-23.3°); from latitude 40° N, the Sun is 16.7° south of the zenith on that day. (For more on Sun angles, see March 15 - 21.)

Since the vernal or spring equinox last March, the Sun has crept ever northward, averaging about one degree every four days, or the apparent width of two Suns side by side. This motion gradually changes the Sun's angle of light with the ground, as well as where sunlight falls in your region of the world. In the Northern Hemisphere, the Sun is very high in the sky at noon on this date; noontime shadows are short. The opposite conditions prevail in the Southern Hemisphere at this time of year.

Perhaps you've noticed sunlight streaming through a window in winter that remains shaded in summer, or that the Sun rises and sets further north of prominent horizon landmarks than it did in March.

The Sun does not move at a constant rate in its north-to-south oscillation. This is particularly obvious by noting its sunrise or sunset positions on the horizon over a period of days around the equinox and the solstice. The Sun moves fastest around the equinox, almost half a degree a day at mid-latitudes. But like a pendulum at the limit of its swing, it slows almost to a complete stop around the time of the solstice, rising at the same place on the horizon for almost a week. The word solstice, in fact, comes from the Latin solstitium, meaning, 'Sun stand still.'

The Sun's motion was closely followed by a number of North American Indian tribes. The Pueblos feared the Sun might cease its back-and-forth swing at either extreme - the summer or winter solstice - creating disastrous consequences for their climate and agriculture. The Hopi relied on the Sun's horizon position to tell them when to plant crops, when to harvest, when to hunt, and conduct weddings and other ceremonies. They kept track of the Sun, and thus the season, by noting its position relative to sharp peaks and mesas on the horizon, in effect, creating a horizon calendar.

The summer solstice has been called the 'noon of the year' because the Sun is at its highest overhead postion for the year (although this reflects a Northern Hemisphere bias; for the Southern Hemisphere, it must be the 'midnight' of the year). In the Northern Hemisphere, the summer Sun is higher in the sky with respect to our horizon, and therefore remains above the horizon for a longer daily period than it does during the winter months.

When we think of the first day of summer, our thoughts likely turn to picnics, trips to the lake or beach, baseball, and general lazing about. Now's the time to get out and enjoy the weather before the first chills of autumn return, which always seems just around the corner for those living in the more northerly latitudes of the world.

Also this week:

• The upper portion of Scorpius, including the bright red star Antares, rises in the southeast for observers at northern latitudes by 9 p.m.

June 21-27

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