Mr. Dacey’s Earth’s Motions Essays
1. Describe our place in the universe, starting on Earth.
Earth is a small, rocky planet, the 3rd from the sun, in an elliptical orbit about 150 million km (1 AU) away. The sun, its planets and their moons, and many smaller objects in solar orbit, are called the solar system. The sun is one of about 400 billion stars in the Milky Way galaxy, orbiting about 35 kly away from the center.
The Milky Way is in a cluster of about 50 galaxies called the Local Group. Two of the smaller galaxies in the Local Group, the Large and Small Magellenic Clouds, are nearby companions of the Milky Way. The nearest large galaxy in the cluster is the Andromeda galaxy, about 2.5 Mly away.
The Local Group is itself part of the enormous cluster of galaxy clusters called the Virgo Supercluster. The Virgo Supercluster is part of an unimaginably vast, expanding web of about 300 billion galaxies that make up the known universe, with a diameter of about 92 billion light years.
2. Describe the relationship between Earth’s motions and the way we measure time.
The day/night cycle was originally used as our standard of time. This cycle is due to Earth’s rotation, with one complete rotation taking a bit less than 24 hours with respect to the stars.
We use the year for longer periods of time, and the year is based on Earth’s revolution, or orbital period, which takes about 365.26 days. We use leap days to align our calendars to make up for the extra .26 days. Every 4 years, Feb. 29 is added to the calendar, except for years ending in 00.
Seasons are also based on Earth’s orbital period, combined with the planet’s axial tilt.
We also divide the year into months, which were originally based on the moon’s 29.5 day cycle of phases.
3. Explain what causes the seasons.
Because the Earth is round, sunlight is more direct near the equator and less direct near the poles. The Earth’s rotational axis is tilted 23.5° from the ecliptic, and when the north pole of the axis is tilted most toward the sun, the most direct sunlight will be 23.5° north or the equator at the Tropic of Cancer. The entire northern hemisphere experiences longer days with the sun highest in the sky at noon on the Summer Solstice in late June. Because of the tilt, the sun doesn’t set on this 24-hour day anywhere north of the Arctic Circle. Seasons are less extreme as you move from the poles toward the equator.
Because the axis always tilts towards the star Polaris, its tilt changes with respect to the sun as Earth orbits (a diagram here may help). Six months after the Summer Solstice, the Earth’s north axis will be tilted away from the sun and with less direct sunlight, winter is colder. The greatest tilt away from the sun for us happens on the Winter Solstice in late December. The seasons are reversed for the southern hemisphere.
Halfway between the solstices are the Vernal (spring) and Autumnal Equinoxes, when the Earth is tilted neither toward nor away from the sun. The length of day and night are equal on the equinoxes, and the sun is directly overhead at noon on the equator. On the Vernal Equinox the sun rises at the North Pole, and doesn’t set until the Autumnal Equinox 6 months later.