The Earth program is a simple model of the earth, the moon and the stars. It is not an exact simulation, but an approximation that illustrates many of the basic principles of astronomy. You can select 4 different views, which allows you to get 4 different perspectives.

• The reason we have seasons is because the axis of the Earth is inclined by 23.5 degrees. The angle remains pointed in the same direction all the time, even as the Earth orbits the Sun. If there were any variation, the next fact would not be true ...
• The north axis of the Earth always points toward the same star in the sky, known as the North Star or Polaris. From the northern hemisphere, the night sky appears to rotate around this point. They rotate around this point every day and also once while the Earth rotates around the Sun.
• The vertical angle of the North Star is equal to your latitude. If your latitude is 35 degrees north, the North Star will be 35 degrees above the horizon. At the equator (0 degrees latitude), the North Star appears on the horizon. At the north pole (90 degrees latitude), the North Star is directly overhead.
• Since 87% of all people live in the northern hemisphere, the southern hemisphere tends to be ignored. People in the southern hemisphere have their seasons opposite from people in the northern hemisphere. The winter in the north is summer in the south and vice versa. However, winters are not as cold because, winter arrives in the south when the earth is closest to the sun.
• People in the southern hemisphere cannot see the North Star. Instead, they can see the sky rotate around the place marked by southern axis of the Earth. Unfortunately, there is no bright star, like the North Star, which marks this point.
• The highest angle of the Sun in the sky depends on your latitude and the time of the year. On June 22, in the northern hemisphere, the positive inclination of the sun is greatest. The highest angle is the sum of 23.5 degrees plus 90 degrees minus your latitude. If your latitude is 35 degrees north, the highest angle of the Sun is 78.5 degrees. At a latitude of 23.5 degrees, the Sun is directly overhead. On December 22, the negative inclination is greatest. At a latitude of 35 degrees north, the highest angle of the Sun is 31.5 degrees.
• The Sun does not rise and set at the same location every day. In Fall and Winter, the Sun rises and sets south of east and west. In Spring and Summer, the Sun rises and sets north of east and west. The Sun rises and sets directly east and west on only two days a year, the Summer Equinox and the Fall Equinox. Ancient people knew this and often aligned monuments so that they would mark this day.
• The Sun generally does reach the highest point exactly at noon. One reason is that many countries impose a daylight savings time which moves the clocks back by an hour. In this case, the Sun will reach the highest point an hour later. Also, there are fixed time zones in which everyone is on the same time. The Sun will reach the highest point at noon only for people in the middle of the time zone.
• The orbit of the Moon is not on the same plane as the orbit of the Earth. If it were, there would be a solar eclipse with every new moon and a solar eclipse with every full moon. Instead, the orbit of the Moon is inclined by 5.145 degrees. Furthermore, this inclination rotates every 18.6 years (the Saros Cycle).
• Viewed from above, everything orbits or rotates in a counterclockwise direction, including the rotation of the Earth and the orbit of the Earth and Moon, and the rotation of the inclination in the Moon orbit.

The Ecliptic View shows the Earth level with the orbit of the Earth around the Sun. This View can be used to demonstrate the inclination of the Earth axis and how that affects the seasons. You can move the slider on the right to move forward to a specific date. The simulation starts at the Winter solstice (Dec 22), where the northern hemisphere is tilted away from the sun and the days are shortest. This causes the northern hemisphere to receive the least heat from the sun in winter. In contrast, the southern hemisphere is enjoying the start of summer and is receiving the most heat and has the longest days. Moving the slider to the Spring Equinox (Mar 22) shows that the inclination of the Earth is now sideways with respect to the Sun. This means that there is no inclination in the direction of the sun, which means that the day and nights are 12 hours each for the entire Earth. Moving the slider to the Summer Solstice (Jun 22) shows how the northern hemisphere is tilted towards the sun and the days are now longest. Thus, the northern hemisphere is now experiencing summer, while the southern hemisphere is now experiencing winter. (However, because the orbit of the Earth is not circular, the Earth is now closer to the Sun, which is why winter in the southern hemisphere is not as cold.) Moving the slider to the Fall Equinox (Sep 22) shows the same results as the Spring Equinox - the days and nights are 12 hours each.

The Celestial View (and the other two views) show the Earth level with the equator. This view shows how things look from the perspective of someone who is on the earth. The paths of the sun and the moon are both tilted with respect to the earth. This is why you don't have a lunar eclipse every time there is a new moon. The only time where an eclipse can happen is when the paths of the sun and the moon cross. The apparent tilt in the path of the Sun is due to the inclination of the Earth. The tilt in the path of the Moon is because the orbit of the Moon is tilted with respect to the Earth orbit around the Sun.

The Midnight View shows the constellations which are visible at midnight throughout the year from a latitude of 35 degrees north. Moving the slider allows you to watch the constellations rotate around the earth. It you look north and up, you will see that the constellations are rotating around a central point (the North Star) which is 35 degrees up. Interestingly, the vertical location of this center of rotation equals your latitude. If you are on the equator, the center is located at 0 degrees, along the horizon. If you are on the north pole, the center is located straight up at 90 degrees. People in the southern hemisphere cannot see this center of rotation. Instead, they see a center of rotation which is located exactly above the south pole. Also note that you can only see certain constellations at certain times of year. They are not gone, but are hidden behind the blue sky that we see during the day.

The One Day View shows you how the sky looks for the 24 hour period from a latitude of 35 degrees north on the day selected by the slider. The red line shows the path of the sun during the day. At the Winter solstice, the sun is lowest overhead and does not travel all the way from east to west. At the Spring and Fall Equinox, the maximum angle of the sun is overhead equals 90 minus your latitude. On the equator, the sun is straight overhead (90 - 0 degrees). At the poles, the sun is at zero degrees (90 - 90 degrees). At night, the visible constellations are rotating across the night sky. As during the year, the axis of rotation is the North Star.