Sunrise

Sunrise, also called sunup, is the special moment when the Sun first appears above the horizon in the morning. It marks the beginning of a new day and is part of the Sun path as the Earth rotates. Watching the Sun rise can be beautiful and gives us light after the dark of night. The term sunrise can also describe the whole time it takes for the Sun to move fully above the horizon.

Terminology

The Sun seems to rise from the horizon, but it is actually the Earth moving that makes it look like the Sun is moving. This idea of a moving Sun led many cultures to create stories and beliefs around the Earth being the center of everything, until astronomer Nicolaus Copernicus showed that the Sun is at the center instead in the 16th century.

Astronomers say sunrise happens for just a moment when the top edge of the Sun first appears on the horizon. But we often use the word "sunrise" to talk about the time before and after this moment. This includes twilight, when the sky gets lighter in the morning but the Sun is not yet visible, and the time after the Sun rises when we still see beautiful colors in the sky.

Measurement

Angle with respect to horizon

Before the Sun actually reaches the horizon, we see something called a false sunrise. This happens because Earth's atmosphere bends the Sun's light, making it look like the Sun is rising earlier than it really is. On average, this bending effect is about 34 arcminutes, but it can change depending on the air conditions.

When we talk about sunrise, we look at the top edge of the Sun, not the middle part. The Sun appears to be about 16 arcminutes wide when it is at the horizon. Because of these two angles — the bending of light and the Sun’s width — sunrise is officially when the Sun’s center is actually 50 arcminutes below the horizon, or about 90.83° from directly overhead, called the zenith.

Time of day

See also: Daytime length variations with latitude and seasons

The time sunrise happens changes throughout the year and depends on where you are on Earth. This is because of several factors: Earth's tilt, its daily spin, its path around the Sun, and the way Earth and the Moon move around each other. We can use something called the analemma to guess when sunrise will happen.

In late winter and spring, sunrise happens a little earlier each day. It gets earliest just before the summer solstice. After that, sunrise happens a little later each day, getting latest shortly after the winter solstice. These dates change a bit depending on where you live.

The bending of light in the atmosphere can also change when we see sunrise. Near the poles, the Sun rises very slowly because it moves across the horizon at a shallow angle.

Location on the horizon

See also: Sun path and Solar azimuth angle

If we ignore the bending of light and the width of the Sun, sunrise in temperate areas happens between northeast and southeast. Exactly at the March equinox and September equinox, the Sun rises due east for everyone on Earth. Calculating the exact direction the Sun rises on other days is tricky, but we can estimate it fairly well using the analemma.

Hemispheric symmetry

An interesting pattern shows up when we look at sunrise and sunset in different parts of the world. This pattern, called hemispheric symmetry, appears when we use the same math to describe the Sun's position in the sky.

Appearance

Colors

See also: Sunset § Colors

Further information: Atmospheric optics

Air molecules and airborne particles scatter sunlight as it passes through the Earth's atmosphere. This scattering changes the colors we see. When the Sun is at sunrise or sunset, the light travels through more atmosphere, so blue and green colors scatter away. This leaves us seeing beautiful orange and red colors.

Sunset colors are often brighter than sunrise colors because the evening air has more particles. Big events like volcanic eruptions can change these colors. Sometimes, they can create amazing red glows just after sunset or just before sunrise.

Optical illusions and other phenomena

• Atmospheric refraction makes the Sun look like it is above the horizon even when it is just below it.
• The Sun looks wider than it is tall when it is near the horizon.
• The Sun looks bigger at sunrise than when it is high in the sky, similar to how the Moon illusion works.
• It looks like the Sun rises and moves around the Earth, but really it is the Earth that is spinning.
• Sometimes a false sunrise can be seen, which is a special kind of light effect called a parhelion.
• Just before sunrise or after sunset, a quick green spot can sometimes be seen above the Sun, called a green flash. This only lasts for a second or two.