June 16, 2004
Alan MacRobert, Senior Editor
617-864-7360 x151, macrobert@SkyandTelescope.com
|Note to Editors/Producers: This release is accompanied by a high-quality animation and illustration; see details below.|
On Sunday, June 20th, at 8:57 p.m. Eastern Daylight Time, the Sun will reach its northernmost point in the sky for the year 2004 and begin its six-month return journey southward. This moment marks the June solstice, the official beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
The seasons’ starting times are governed by the Earth’s motion around the Sun or equivalently, from our point of view, the Sun’s annual motion in Earth’s sky. The start of summer (for the Northern Hemisphere) is defined as the moment when the Sun hovers over Earth’s Tropic of Cancer (the line of latitude 23½° north of the equator) before heading south a moment called, by Northerners, the summer solstice.
The Sun appears to move north and south in our sky during the year because of what some might consider an awkward misalignment of our planet. Earth’s axis is tilted with respect to our orbit around the Sun. So when we’re on one side of our orbit, the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. When we’re on the opposite side of our orbit, the Northern Hemisphere is tipped away from the Sun. The solar rays come in at a lower slant to this part of the world and heat the ground less, making winter.
North of the equator, the June and December solstices mark the beginning of summer and winter, respectively. The effect is opposite for inhabitants of the Southern Hemisphere; for them the June solstice signals the beginning of winter, while summer starts at the December solstice.
For a skywatcher on Earth (at north temperate latitudes), the effect is to make the Sun appear to move higher in the midday sky each day from December to June, and back down again from June to December. A solstice comes when the Sun is at the upper or lower end of its journey; an equinox comes when the Sun is halfway through each journey.
The word solstice comes from the Latin solstitium sol meaning "sun" and stitium "stoppage." The summer solstice marks the longest day and shortest night of the year. It is also the time when the days begin to grow shorter and the nights longer.
The summer solstice is sometimes called midsummer. Ancient peoples used to gather at midsummer to celebrate the life-giving power of the Sun. Even today thousands still flock to Stonehenge, the ancient stone circle on England's Salisbury Plain, to greet the dawn on the summer solstice. As seen from inside the inner circle of large stones on this day, the Sun rises over the Heel Stone, which lies outside the main ring.
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