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Tuesday, July 22, 2014

Sun's Angle Lower & Shadows Longer Since First Day of Summer

Even though we're just a little more than a month removed from the Summer Solstice, there are subtle signs that the "longest day of the year" was just over four weeks ago. You may have noticed that the days are getting shorter, since the Sun rises 19 minutes later and the Sun sets 11 minutes earlier than it did on the first day of Summer. However, less discernible is the change in the length of shadows. Believe it or not, the shadows have been slowly getting longer, and that's due to the Sun's lower angle in the sky.

The Sun reached its highest angle in the sky --- or declination --- in the Northern Hemisphere on the first day of Summer. The declination of the Sun is the measurement of the angle between the Sun’s rays and the Earth’s equatorial plane. The Earth’s axis is tilted by 23.5 degrees away from the solar plane. This explains why we have different seasons and why the Northern Hemisphere and the Southern Hemisphere always have contradicting seasons. When tilted towards the Sun, it's Summer in the Northern Hemisphere.

The Sun’s declination varies throughout the year. Its declination becomes zero during the Spring Equinox and reaches the maximum declination angle of 23.5 degrees during the Summer Solstice. It reverts to zero declination when the Fall Equinox occurs and drops to a negative 23.5-degree declination during the Winter Solstice.

Today, for example, the declination of the Sun is an 20.24 degrees North of the celestial equator. That's more than three degrees lower than where it was June 20. By the end of the month, it will be 18.5 degrees North of the celestial equator. The Sun's declination will be +14.15 degrees by August 15, and +8.52 on the final day of next month. By September 16, it will be just 3.16 degrees North of the celestial equator. As you can see, the Sun's angle will be dropping lower in the sky over the next two months.


The change in the Sun’s declination results in yearly cycles which are observed as each season progresses. The declination of the Sun has effects on its own altitude and to the duration of daylight. The Sun reaches its highest altitude above the horizon each day at noon in the Northern Hemisphere. With respect to the celestial equator, it reaches the highest altitude of 73.5 degrees the first day of the Summer, while its altitude reaches the minimum 26.5 degrees during the first day of Winter.

Yesterday marked one month since the Summer Solstice, and the Autumnal Equinox is two months away. Hot and humid weather is expected through tomorrow before a cold front arrives, triggering showers and thunderstorms. Much more comfortable weather will be with us later Thursday into Friday.

Paul