Seasons, Tides, and Lunar Phases

Seasons, Tides, and Lunar Phases — page 1 — In this long photo exposure you can see how the rotation of the Earth makes it appear as though the stars travel in circles around the poles The Sun and the Seasons Stand in one place and look up at the sky It probably feels like you’re not moving at all In reality you’re zooming along with Earth at 18 5 miles 30 kilometers per second around the sun At the same time the Earth itself is spinning at 1,040 miles 1,674 kilometers per hour It’s amazing you aren’t dizzy But just as you cannot feel yourself moving at 60 miles 97 kilometers per hour in the car you cannot feel the movement of the Earth Gravity keeps you grounded and your momentum or speed matches our planet’s But you still experience the effects of all that movement It takes 24 hours for the Earth to complete one complete spin or rotation It’s 4

Seasons, Tides, and Lunar Phases — page 2 — daylight for those on the side of the Earth facing the sun and night for those on the other side The Earth’s spin gives us day and night but Earth’s year-long journey around the sun gives us the seasons The Earth is not perfectly parallel to the sun The Earth’s axis has a slight tilt of 23 5 degrees because the extra land at the North Pole makes the Earth top-heavy The Northern Hemisphere is tilted away from the sun for part of the year and toward the sun for another part As the tilted Earth travels around the sun the angle of the sun’s rays hitting the planet changes and causes the seasons What’s a Leap Year It takes the Earth a year to revolve around the sun Each revolution lasts 365 25 days Every four years each quarter of a day adds up to a single day In those leap years the year has 366 days with the extra day on February 29 The North Star and the Southern Cross As the Earth travels around the sun its place in the solar system shifts That changes our view of the stars Constellations that are out in summer will dip below the horizon in the winter and vice versa Meanwhile the rotation of the Earth also changes the placement of the stars in the sky But you’ve probably heard that the North Star named Polaris is always in the northern part of the sky How can that be The Earth tilts toward Polaris and the Earth’s tilt never changes so the North Pole always faces the North Star When we look toward the North Pole we’re always looking toward Polaris A similar star signpost is in the south called the Southern Cross or Crux The five stars that make up the Southern Cross guided explorers as they sailed toward Australia and New Zealand in the Southern Hemisphere 5

Seasons, Tides, and Lunar Phases — page 3 — AUTUMN Equinox September 22-23 WINTER Solstice December 21-22 SUMMER Solstice June 21-22 SPRING Equinox March 19-20 While the Northern Hemisphere is tilted toward the sun the sun’s rays called solar radiation hit the Northern Hemisphere more directly That more direct angle means more heat giving us summer At the same time solar radiation reaches the Southern Hemisphere less directly The rays travel farther and lose more heat creating winter in the south Six months later when the positions are reversed the Southern Hemisphere enjoys summer while it’s winter in the north 6

Seasons, Tides, and Lunar Phases — page 4 — Twice a year the sun shines Spring and Autumnal Equinox Days directly on the equator as it passes from the Northern Hemisphere to the Southern Hemisphere and then back again Each of these days is called an equinox The spring equinox or vernal equinox for the Northern Hemisphere occurs around March 19-20 when the sun passes from the south to the north The autumnal equinox occurs around September 23 when the sun crosses from the north to south The Northern Hemisphere’s spring equinox is the Southern Hemisphere’s autumnal equinox The farther from the equator you get the lower the winter sun will sit in the sky because it travels a shorter distance across the sky You might have noticed the days get longer in the summertime and shorter in the wintertime That’s because the sun follows a longer path higher in the sky when it’s more directly opposite the Earth in the summer The longest day of the year is the summer solstice occurring between June 20 and 22 in the Northern Hemisphere The winter solstice is the shortest day of the year when the sun follows the shortest path across the sky Winter solstice occurs around December 21 to 22 The equator running along the middle of the Earth gets similar amounts of sunlight all year long because the angle of the sun’s rays is similar Seasons at the equator are also mild equator 7

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