Astronomy 04 The Solar System Chapter 4: “Earth, Moon & Sky"”

What causes the seasons? Are the seasons caused by Earth’s changing Distance from the Sun?

Earth’s orbit is elliptical not circular and the Sun is not in the center but at one focus of the ellipse. Earth is closest to the Sun or at perihelion – about 91 million miles away - on January 3. Earth is farthest from the Sun or at aphelion – about 95 million miles away – on July 4. Meteorology 10: Weather Processes The seasons are not caused by Earth’s distance from the Sun. The Earth orbits the Sun once a year or every 365.25 days. The Earth’s orbit is not a circle but an ellipse. It is closest to the Sun or at perihelion – about 91 million miles away - on January 3. It is farthest from the Sun or at aphelion – about 94 million miles away – on July 4. The seasons are caused by the tilt of Earth’s axis.

The seasons are not caused by Earth’s distance from the Sun but by the tilt of Earth’s axis.

The summer solstice is the time (Jun 21) when the north pole of Earth points most directly at the Sun. The winter solstice is the time (Dec 21) when the north pole of Earth is pointed most directly away from the Sun.

The vernal and autumnal equinoxes are times when the polar axis of Earth is midway between the extremes reached at the solstices.

The Moon travels around the Earth once every 27 1/3 days. This cycle is know as the Sidereal Period.

As the Moon orbits Earth, it displays different phases which repeat every 29 ½ days. This cycle is known as the Synodic Period.

New Moon occurs when the Moon is between the Earth and Sun. No moon can be seen in the sky at this time.

First Quarter Moon occurs when the Moon is at 90 degree angle with the Earth and Sun. We see a half Moon at this time.

Full Moon occurs when the Moon is directly opposite the Sun as viewed from Earth. We see the fully illuminated hemisphere of the Moon at this time.

Third Quarter Moon occurs when the Moon is again at a 90 degree angle with the Earth and Sun. We see a half Moon at this time.

The Moon rotates on its axis in exactly the same amount of time it takes to orbit Earth (27 1/3 days). This is called synchronous rotation.

The gravitational pull of the Sun and Moon on Earth produces ocean tides. There are 2 high and 2 low tides daily.

Figure 4.16 Pull of the Moon. The Moon’s differential attraction is shown on different parts of Earth. (Note that the differences have been exaggerated for educational purposes.)

Figure 4.17 Tidal Bulges in an “Ideal” Ocean. Differences in gravity cause tidal forces that push water in the direction of tidal bulges on Earth.

Figure 4.18 High and Low Tides. ofThis is a side-by-side comparison of the Bay Fundy in Canada at high and low tides. (credit a, b: modification of work by Dylan Kereluk)

Figure 4.19 Tides Caused by Different Alignments of the Sun and Moon. In spring tides, the Sun’s and Moon’s pulls reinforce each other. In neap tides, the Sun and the Moon pull at right angles to each other and the resulting tides are lower than usual.

Spring Tides are highest and lowest tides that occur at times of Full and New Moons. Neap Tides are next highest and lowest tides which occur at times of the First and Last Quarter Moons.

Figure 4.21 Solar Eclipse. (a) The shadow cast by a spherical body (the Moon, for example) is shown. Notice the dark umbra and the lighter penumbra. Four points in the shadow are labeled with numbers. In (b) you see what the Sun and Moon would look like in the sky at the four labeled points. At position 1, you see a total eclipse. At positions 2 and 3, the eclipse is partial. At position 4, the Moon is farther away and thus cannot cover the Sun completely; a ring of light thus shows around the Sun, creating what is called an “annular” eclipse.

A solar eclipse occurs when the Moon is at its New Phase and at one of the nodes in its orbit, the New Moon casts a small shadow on Earth. Within this shadow observers see a Solar Eclipse as the Moon block the Sun from view.

Lunar and solar eclipses dn’t occur every New and Full Moon because the orbit of the Moon around the Earth

Features of a Total Solar Eclipse: Sky darkens to deep twilight. Planets and stars come into view. Corona, chromosphere and prominences appear around Sun. Bailey’s Beads or a Diamond Ring may be seen at the beginning and end of totality.

Total Eclipse of the Sun

Figure 4.23 The Sun’s Corona. The corona (thin outer atmosphere) of the Sun is visible during a total solar eclipse. (It looks) more extensive in photographs than it would to the unaided eye.) (credit: modification of work by Lutfar Rahman Nirjhar

A lunar eclipse occurs when the earth’s shadow almost completely blocks the Moon from view A solar eclipse occurs when the Moon blocks the Sun from view. Solar or lunar eclipses can be partial or total.

A lunar eclipse occurs when the Moon is at its Full Phase and at one of the nodes in its orbit, the Full Moon passes through Earth’s shadow and a lunar eclipse occurs on Earth.

The Moon does not usually disappear from view during an eclipse but turns a rosy red color. Lunar eclipses are always visible over large areas of Earth and for period of several hours.

The Diamond Ring Effect

Chapter 3: “The Cycles of the Moon If, in addition to the above two circumstances, the Moon is at apogee in its orbit, places on Earth will experience an Annular Eclipse. If, in addition to the above two circumstances, the Moon is at perigee in its orbit, places on Earth will experience the longest possible duration of a Total Solar Eclipse – 7 ½ minutes ! Solar eclipses are always visible over very small areas of Earth and for very short times.

Annular