Latest star trails
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Reading Star Trails
Why do the stars appear to move across the sky? Discover how Earth's rotation creates star trails and what they can reveal about the night sky above Billingborough.
STARTING WITH THE BASICS
What is a star trail?
If you take a photograph of the night sky with a long enough exposure, the stars do not necessarily appear as points of light. Instead, they can appear as lines or arcs stretching across the image.
These are known as star trails.
The stars themselves are not suddenly travelling across the sky in enormous arcs. The apparent movement is mainly caused by the rotation of the Earth as it turns beneath the stars.
The sky appears to move because the Earth is rotating.
Star trails provide a visible record of that apparent motion, turning something that normally happens too slowly for us to notice into a striking image.
BILLINGBOROUGH OBSERVATORY
A night recorded above Lincolnshire
Our AllSky camera can combine observations from throughout the night to produce a star-trail image showing the apparent movement of the stars above the observatory. Of course, if it has been overcast, the image will be just clouds.
Latest star-trail image from the Billingborough Observatory AllSky camera.
EARTH'S ROTATION
The sky appears to turn
The stars are so distant that, over the course of a human lifetime, their positions appear almost fixed relative to one another. This is why we often think of the night sky as a permanent backdrop.
But the Earth is rotating once approximately every 24 hours. As we rotate, the entire celestial sphere appears to move across the sky.
Over a short period this movement is difficult to notice with the unaided eye. Over several hours, however, the effect becomes obvious in a photograph.
A long observation makes slow movement visible.
The camera does not need to move. The changing position of the stars is recorded from a fixed viewpoint as the Earth rotates.
LOOKING AT THE SHAPE
Why are the trails curved?
The apparent movement of the stars is not the same everywhere across the sky. Stars appear to move around the celestial poles, creating curved paths when their movement is recorded over time.
Stars close to a celestial pole appear to trace relatively small circles. Stars farther from the pole appear to travel along much larger arcs.
The result is the familiar pattern of concentric-looking trails around the region of the celestial pole.
Because the AllSky camera has such a wide field of view, it can capture a large part of this pattern in a single image.
FINDING YOUR WAY AROUND THE SKY
The celestial pole
The Earth's rotation axis points towards two locations in the sky known as the celestial poles: one in the northern hemisphere and one in the southern hemisphere.
For an observer in Lincolnshire, the northern celestial pole is above the northern horizon. The apparent paths of the stars curve around this point during the night.
This provides an important clue when interpreting a star-trail image: the direction and curvature of the trails can tell us something about which part of the sky the camera is seeing.
Star trails can act like a celestial compass.
The pattern of the trails can help reveal the direction in which the camera is pointing and the position of the celestial pole within the image.
FROM MANY IMAGES TO ONE
How Billingborough creates a star-trail image
The AllSky camera records individual images throughout the night. Each photograph is a snapshot of the sky at that particular moment.
The images can then be combined so that the stars recorded in successive frames build into continuous trails.
This is different from simply taking one extremely long exposure. By using many individual images, the resulting observation can preserve a much greater amount of information about what happened throughout the night.
Thousands of individual observations can become one image.
The finished star-trail photograph is therefore a visual summary of the movement recorded throughout the observing period.
READING THE OBSERVATION
What can we learn from star trails?
A star-trail image is more than an attractive photograph. It can provide information about the movement of the sky and the conditions during the observation.
The trails can show the apparent direction of stellar movement, while their curvature provides clues about the position of the celestial pole.
The image can also reveal interruptions in the observation. Clouds, bright sources of light and other changes in the sky may leave their own signatures in the combined image.
When the image is considered alongside the original sequence of observations, it becomes possible to understand not just what the finished picture looks like, but what happened during the night that produced it.
A LITTLE PERSPECTIVE
What are we actually seeing?
Star trails are an example of apparent motion. The stars are not physically travelling around the Earth in the circular paths shown by the trails.
Instead, the pattern is produced because we are observing the celestial sphere from a rotating planet.
The same principle explains why the Sun, Moon and planets appear to cross the sky during the day or night. The rotation of the Earth changes our viewpoint as time passes.
We are watching the Earth turn.
A star-trail image turns Earth's rotation into something we can see directly.
KEEP EXPLORING
Explore the observations
The AllSky camera continues to record the sky above Billingborough Observatory every night.
Understanding the AllSky Camera
Discover how the camera observes the entire sky and turns individual images into useful observations.
Explore the AllSky camera →More astronomy
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