# AstroLab: How to Blink Images and Measure Distance

Use the video and text guides to learn how to use AstroLab to blink through several images to find things that change. Then use this to measure the distance an object has moved.

If you are brand new to AstroLab, you first need to know how to view your images.

To quickly flick between images when you have several open you can either use the arrow keys at the bottom of the image boxes, or use the shortcut.
To use the shortcut press the "CONTROL" button on your keyboard and the number "2" to quickly skip forward an image. Use "CONTROL + 1" to skip back. If you hold these buttons down it will quickly move through all of the images you have open and then circle around again.

## Measuring Distance

It is possible to measure the distance an object has travelled between images.

1. First check the pixel coordinate where the object started on the first image.
2. Then use "CONTROL 2" to move to the last image of your object.
3. Select the "Measure Size" tool in the "Astro" menu to see objects move over time.
4. Select the option to add a line.
5. Place one end of the line on the pixel position where the object started.
6. Change the size and shape of the line until the other side is on the object's current position.
7. The "Measure Size" box will show you the size of your shape in pixels.

## Convert your size from pixels to a unit of distance

Pixels are not a very useful unit of measurement in this instance. It is telling you how much of the image your shape takes up, not the true size of it. However, you can convert the measurement to a useful unit of distance using these steps:

1. Note down the measurement in pixels.
2. Note down the distance each pixel equals and the units (this will probably be in km).
3. Multiply the values together to calculate the size of the object in those units.

Please note, the distance each pixel equals depends on what object you observed. It will not always be the same. It is also only true at the distance of your object, it will not be true for any objects in the foreground or background.

## Convert your distance to a speed

You can change the distance to speed by knowing the time taken between your observations and then using the equation: speed = distance / time

1. Open the "Observation Details" tool in the "Astro" menu
2. Note down the time the first observation was taken
3. Note down the time the last observation was taken
4. Work out the difference between these times (this will probably be in hours)
5. Divide the distance you calculated before by this value
6. This is your speed (likely in units of km per hour)