Air mass (astronomy)

When we look at stars, light must travel through air.

Scientists use a way to measure this air. They call it air mass.

If a star is directly above you, the light travels through less air. This makes the star look bright.

If a star is low near the ground, the light travels through much more air. This makes the star look dim.

Looking through more air can scatter the light. This happens as the light moves through the thick air.

At the horizon, the air mass is very high. It can reach a value of about 38. This helps us know how stars will look.

When we look at stars, light must travel through our air. Scientists use a measure called air mass to study this. Air mass tells us how much air the light passes through.

Imagine the air is like a thick blanket. If a star is directly above you, the light travels a short way. This point is called the zenith. At the zenith, the air mass is exactly 1. The star looks its brightest here.

But stars can also be low near the horizon. To reach you, the light must travel through much more air. This makes the star look dimmer. This dimming is called atmospheric extinction. It happens because the air scatters and absorbs the light.

As the angle from the zenith grows, the air mass grows too. Near the horizon, the air mass can reach about 38.

Air can also bend the light. This is called refraction. Refraction makes stars look higher in the sky than they really are. Because of these changes, calculating air mass near the horizon is very hard for scientists.

When astronomers look at stars, they must look through Earth's atmosphere. This layer of air changes how we see space. Scientists use a measurement called air mass to describe this. Air mass tells us how much air a light ray passes through.

This measurement is very important for seeing things clearly. It helps us understand why some stars look brighter than others.

Light travels through the air in a specific way. As light enters the atmosphere, it hits air particles. This causes the light to scatter or be absorbed. This process is called atmospheric extinction.

When a star is at the zenith, it is directly overhead. At the zenith, the air mass is exactly 1. If a star is near the horizon, the light travels through much more air. This makes the star appear less bright.

Many people have worked to find ways to calculate air mass. Different scientists have created different formulas to help. For example, Allen created an abbreviated table of values. Other researchers like Kasten and Young developed formulas to help.

Some formulas work well for stars high in the sky. However, calculating air mass near the horizon is a very hard job. This is because the air is thicker and more complex there.

There are many specific numbers used to study these paths. At the zenith, the relative air mass is always 1. As you move toward the horizon, the air mass grows. It can reach a value of about 38 at the horizon.

Some models assume the atmosphere is a simple, flat layer. In those models, the air mass at a 60 degree angle is about 2. Scientists also use the Earth's radius, which is 6371 km, in their math. These numbers help make the models more accurate.

Understanding air mass is like looking through a swimming pool. If you look straight down, the water is thin. If you look at an angle, you see much more water. The atmosphere works in a similar way for light. Light also bends as it enters the air. This bending is called refraction. Refraction makes stars look higher in the sky than they really are. This makes the path of the light slightly longer. Scientists must include this extra length when they do their work.

In astronomy, observing the stars requires looking through Earth's atmosphere. Air mass is a measurement used to describe the amount of air a light ray passes through along its line of sight. This measurement is vital because the atmosphere affects how we perceive celestial objects. As light travels from a star toward an observer, it undergoes attenuation. Attenuation is the reduction in the intensity of light. This happens because light is lost to scattering and absorption as it hits particles in the air.

This process of light loss is known as atmospheric extinction. It is described mathematically by the Beer–Lambert law.

To understand the mechanism, we must look at the path of the light. When a celestial body is at the zenith, it is directly overhead. At this point, the light travels through the shortest possible path. The relative air mass at the zenith is defined as exactly 1. As the object moves toward the horizon, the light must travel through a much thicker layer of air. This increase in path length leads to greater attenuation. Consequently, celestial bodies near the horizon appear less bright than those near the zenith.

The air mass increases as the angle from the zenith increases, reaching a value of approximately 38 at the horizon.

Astronomers use different models to calculate these values. A simple model is the plane-parallel atmosphere. This model assumes the atmosphere is a homogeneous, flat layer where density is constant. In this scenario, the air mass is simply the secant of the zenith angle. For example, at a zenith angle of 60 degrees, the air mass is approximately 2. However, this model is only accurate for zenith angles up to about 60 or 75 degrees. It fails near the horizon because it ignores the curvature of the Earth.

More complex models account for the Earth's spherical shape. A nonrefracting spherical atmosphere model can treat the atmosphere as being concentrated in the lower 9 km of height. In a homogeneous spherical model, the air mass can be calculated using the Earth's radius, which is 6371 km. These models help scientists understand how the density of air changes with elevation. In a real atmosphere, density is not constant; it decreases as you move higher above sea level. Scientists also use isothermal models, where the atmosphere has a constant temperature.

History shows that many researchers have worked to refine these calculations. Allen created an abbreviated compilation of air mass values from earlier sources. Other scientists, such as Kasten and Young, developed formulas to help with these measurements. Different formulas have been designed to fit specific needs. Some work well for high zenith angles, while others attempt to be accurate near the horizon. However, calculating air mass near the horizon is extremely difficult. This is because low-altitude extinction is heavily affected by the concentration of aerosols and local temperature gradients.

Refraction is another critical factor in these calculations. Atmospheric refraction causes light to follow an approximately circular path rather than a straight line. This makes the actual path through the air slightly longer than a geometric straight line. Refraction also causes a celestial body to appear higher in the sky than it actually is. At the horizon, the difference between the true zenith angle and the apparent zenith angle is about 34 minutes of arc. Because of this, many formulas are based on the apparent zenith angle rather than the true one.

Ultimately, air mass connects the physics of light to the structure of our planet. By understanding the density, temperature, and pressure of the atmosphere, astronomers can correct their observations. This allows them to see the true brightness of stars. Whether using a simple plane-parallel model or a complex layered model, the goal remains the same. Scientists want to account for every bit of air that sits between their telescope and the vastness of space.