Table of Contents

Archaeoastronomy
Equitorial Coordinates
Understanding the Seasons
Questions

Intro to Astronomy
Misconceptions

Time & Its Measurement

Telescopes  

Solar & Lunar Eclipses

The Solar System

The Earth

The Moon

Mecury, Venus, Mars

The Outer Planets

Solar System Debris

The Sun

Evolution of Stars

Intersteller Matter

Sky Literacy






Understanding the Seasons

    The reason that the weather is warmer in summer and colder in winter for the northern hemisphere is not due to the fact that the earth is nearer to the sun in summer and farther from the sun in winter. In fact, just the opposite is true. The seasons are the result of the differing amounts of solar energy which are received at the earth's surface. This changing insolation is a result of the 23 ½° inclination of the earth's axis to the perpendicular of the ecliptic, and the revolution of the earth around the sun. The earth's axis is the imaginary line about which the earth rotates. Expressed in another way, the ecliptic (path of the sun in the sky) is tilted to the plane of the celestial equator by 23 ½°. This causes the sun to not only move eastward as the earth revolves around this star, but to also move northward or southward with respect to the celestial equator. This change in the declination of the sun results in three yearly cycles which can be readily observed as the seasons progress:

  1. The altitude of the sun changes: The sun reaches its highest position above the horizon at local noon each day. In Allentown, when the sun is at its most northerly position, with respect to the celestial equator, its altitude at noon is at an extreme, 73 ½°. This occurs on the first day of summer. On the first day of winter, the sun is as far south of the celestial equator as it can move and achieves its minimum altitude of 26 ½° for noon as seen from Allentown.
  2. The duration of daylight changes: The longest day of the year occurs for residents of the northern hemisphere when the sun is at its most northerly position with respect to the celestial equator. This marks the time when summer begins. In Allentown, the sun rises in the northeast taking approximately 15 hours before setting in the northwest. The path which the sun takes from rise to set is longest at summer solstice. Therefore the day must also be at its longest, because the earth rotates at a uniform rate. When the sun is at its greatest deviation south of the celestial equator, the day is the shortest. This marks the first day of winter. In Allentown, the sun rises in the southeast, and about nine hours later, it is setting in the southwest.
  3. The positions of sunrise and sunset change: For observers in Allentown, the sun rises as far to the north of east and sets as far to the north of west when it is at its maximum declination north of the celestial equator. When the sun is as far to the south of the celestial equator as it can move, on the first day of winter, it rises as far to the south of east and sets as far to the south of west as it can for that location.

    In winter, the northern hemisphere is tilted away from the sun. The daily duration of sunshine is restricted, and the sun is lower at noon. The sun's energy strikes the ground at a shallower angle, and thus less energy is received per unit area. The temperature becomes colder. In summer, the northern hemisphere is tilted toward the sun. Not only is the daily duration of sunshine longer, but the sun also climbs to a higher altitude, so that its energy strikes our location more directly, and we receive more energy per unit area. It's time to party in the warmer weather.

    The fact that the weather is warmer in the northern hemisphere in summer and colder in winter is NOT because the sun is nearer to us in summer and farther from us in winter. In actuality, the summer sun is three million miles farther from the earth than its winter distance. The two factors which produce the seasonal effects are (1) the duration of sunlit hours experienced during each season and (2) how directly the sun's rays strike the earth's surface. These two factors are in essence due to a single factor, namely the inclination of the plane of earth's equator with the plane of its orbit around the sun. Put another way, the axis of the earth is inclined 23 from the perpendicular to its orbit.

    In summer, the northern half of the earth's axis is tipped toward the sun. Not only is the duration of daylight longest at any place in the northern hemisphere, but the sun transits the meridian at a higher altitude, so that its rays are more nearly vertical and more concentrated. In winter, the northern hemisphere of the earth is tilted away from the sun. The duration of daylight is shorter and the sun transits the meridian at a lower altitude. The sun's rays strike the earth more obliquely and thus impart to the surface of earth less energy per unit area.
    As a result of the earth's revolution around the sun, the sun appears to move eastward along the ecliptic approximately one degree per day. The 23 ½ ° tilt of the earth's equator to the plane of its orbit causes this eastward motion to simultaneously have a northward or a southward component depending upon the season. This causes the sun to be north of the celestial equator for half of the year and south of the celestial equator for the other half. The sun's declination changes by 47° This yearly shift is twice the inclination of the celestial equator to the ecliptic. The variation in declination causes the sun's daily path across the sky to change. This creates a variation in the length of time that the sun is above the horizon.