Photo by: ESO Creative Commons

What do Christian Doppler and Albert Einstein have in common? Simple, they are both instrumental in weather forecasting through their contributions to the information people know about radial velocity. Without understanding how radial velocity works, warnings about tornadoes and hurricanes are not possible.

By simple definition, radial velocity refers to the speed of any object in the direction of an unmoving observer’s line of sight. This can pertain to directions of either moving toward or away from the observer. In a three dimensional space, radial velocity can apply to any object with respect to the observer since the position of the observer remains fixed. However, radial velocity best applies to celestial bodies such as planets.

Planets have two types of velocities. As the planet moves away from the observer when orbiting toward the far side of sun, it is said to have a positive radial velocity. On the other hand, when the planet moves from the far side of the sun toward the observer it is now said to have negative velocity.

This is also where the Doppler shift comes into play. Astronomers observing orbiting bodies use data in the form of electromagnetic waves detectable from telescopes. The Doppler shift is seen as electromagnetic waves are compressed and possess a higher frequency when objects move toward the observer and change into a lower frequency when moving away from the observer.
Aside from using radial velocity to gauge the orbital cycle of planets, it can also be used to indicate the rotational movement of storms.

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Photo by: Tamasflex Creative Commons

If you recently have had a conversation with an astronomer, then you’ve probably heard them mention the exit pupil as a certain eye piece in telescopes or binoculars. But what does exit pupil really mean, and how can it affect views when looking through a telescope?

By definition, exit pupil is the image of an object projected by the eye piece. Thus, all light passes through the exit pupil, and it should ideally be the same size as the pupil of your eye. Any millimeter larger can be harmful.

The exit pupil is calculated as the objective diameter partitioned by magnification. To find your binoculars’ or telescope’s exit pupil, determine the focal length of the instrument and divide it with the focal length of your eye piece. This will give the magnification of the telescope. Then divide the objective diameter by the magnification. Make sure to do this in the same unit, in millimeter.

An exact exit pupil for your telescope or binoculars means that it will provide a more detailed view of objects. You can even see the faintest objects in the farthest horizon. Exit pupils larger than 5 mm will tend to wash out detail and objects will be faint.

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