Earth goes around the Sun once a year. That is about 365 days. Earth is also tilted, so it does not stand up straight. This tilt gives us the seasons. Summer days have more hours of sunlight. Winter days have fewer hours of sunlight.

In June the northern half of Earth is tilted toward the Sun. In December it is tilted away from the Sun. When the north part leans toward the Sun, the south part leans away from it. That is why the two halves have opposite seasons. When it is summer in the north, it is winter in the south.

Two days each year have special names. The summer solstice is the longest day of the year in the north part of Earth. The winter solstice is the shortest day and the longest night.

Two other days are special too. At an equinox, day and night are about equal in length. The word equinox comes from Latin words that mean equal night. The spring equinox is usually on March 20, and the fall equinox is usually on September 22.

You may think summer comes when Earth is closer to the Sun. In fact, in northern summer Earth is not closer. The distance is not what makes summer hot or winter cold.

So what does the tilt do? In summer the Sun is higher in the sky. Its rays hit the ground more directly. When rays hit the ground at a slant, they spread out over more ground. They warm it less. That is part of why winter is cold.

As Earth goes around the Sun, its tilt always points the same way in space. Earth does not wobble back and forth each year. It is the same tilt, about 23 and a half degrees, all year long.

The seasons change living things too. In fall, many tree leaves lose their green color and show yellow, orange and red. Some animals migrate when the seasons change. They travel a long way to a winter home.

Words to know

• solstice: extreme day
• equinox: equal night
• tilted: leaning over
• migrate: travel far