Did you know that you can measure the distances between objects in the sky with just your hands and fingers? To get started, hold your hands at arm's length in front of your face. Now raise your little finger. The width of the tip of your little finger at arm's length is about one degree. So if you are looking for an object that is one degree away from Jupiter, for example, place your little finger next to Jupiter and you get the approximate distance to the object. Raise your three middle fingers to measure 5 degrees. Clench your fist or hold your hand like a policeman stopping traffic to measure 10 degrees. Now stretch your thumb and little finger as wide as you can (one full hand-span) to measure about 20 degrees.



Measuring degrees with your hands


The great thing about this trick is that it will work no matter how young or old you are because the length of your arm is proportional to the size of your hand. Our conventional methods of measuring distances between two objects on Earth make very little sense when measuring the vast distances between celestial objects. Measuring Angles Because of this, astronomers measure the distance between celestial objects based on the angle they make with an observational point on Earth. Known as angular distances or angular separation, distances are expressed in terms of degrees (°), arc minutes ('), and arc seconds ("). While angular separation primarily describes the apparent distance between celestial objects, as seen from Earth, it can also be used to suggest their actual distance from one another. Imagining Angles in the Sky Like our modern-day timekeeping, the angular method is based on a sexagesimal system – a numeral system with 60 as its base. There are 360° in a circle or sphere, each degree is divided into 60' and each arc minute is further divided into 60". Imagine the sky as a big hollow sphere with the Earth at its center – let’s call this the celestial sphere. This sphere is 360°. If you look above at the sky, you will see only half of the sky i.e. 180° of the celestial sphere.

The celestial sphere is an imaginary sphere that is used as a tool for understanding the positions and movements of celestial objects as observed from Earth. It is a concept used in astronomy and is a fundamental tool for celestial navigation and mapping the sky. The celestial sphere is centered on the Earth, and all celestial objects such as stars, planets, the Sun, and the Moon are considered to be located on its surface. From the perspective of an observer on Earth, it appears as though these celestial objects are fixed onto the inside surface of the celestial sphere. The celestial sphere is divided into several important components. The celestial equator is an imaginary line that is a projection of Earth's equator onto the celestial sphere. It divides the celestial sphere into the northern and southern celestial hemispheres. The celestial poles are the points where the Earth's rotation axis intersects the celestial sphere. The North Celestial Pole is located approximately above the Earth's North Pole, while the South Celestial Pole is located above the Earth's South Pole. Additionally, the celestial sphere is divided into various coordinate systems to help locate celestial objects. The most commonly used coordinate system is the equatorial coordinate system, which uses declination (similar to latitude) and right ascension (similar to longitude) to specify the positions of objects on the celestial sphere. Other coordinate systems, such as the ecliptic coordinate system and the galactic coordinate system, are also used for specific purposes. The concept of the celestial sphere simplifies the study of celestial objects by providing a fixed reference frame against which their positions and movements can be measured. It allows astronomers to describe and predict the positions of objects in the sky, as well as determine their rising, setting, and transit times.




  The upper hemisphere of the celestial sphere

The other half of the celestial sphere is below the horizon and cannot be seen. The point right above you in the sky is the zenith. The zenith is always 90° from the horizon. A “Handy” Way to Measure Distances Hold your hand at arm’s length and close one eye. Make a fist, with the back of your hand facing you. The width of your fist will approximately be 10 degrees. This means that any two objects that are on the opposite ends of your fist will be 10 degrees apart. The North Star (Polaris) and Dubhe, one of the northern pointers of the Big Dipper are 3 fists apart. This means that angular distance or angular separation between the two stars is 30°.

Open up your fist, stretch your little finger and thumb as far as you can and curl down the rest of your fingers. The tip of your little finger and your thumb will span about 25°. The Big Dipper spans around 25°. The tip-to-tip span between your index finger and your little finger is 15°. Your three middle fingers will span about 5°. Your little finger at an arms length is about 1° wide. It is important to note that such measurements are approximate – not everyone has the same sized hand. With these simple measuring rules in your hand, you can not only understand basic stargazing jargon but also tell other budding stargazers where to look for a specific celestial object in the sky.Angular Size Angular size or angular diameter of a celestial object is the angular separation between opposite edges of the object. The Sun and the Moon are the only objects in the sky whose angular size is visible to the naked eye.

Remember to never look at the Sun directly without any eye protection! The angular diameter of a full Moon is about 30', while the angular diameter of the Sun is around 32'. Find Your Latitude If you are in the Northern Hemisphere, you can use your hands to find your current location’s latitude. To do this, stretch your hands in front of you and measure the angle between the visible horizon and the North Star.

This angle is your latitude in degrees. Unfortunately, there is no bright star equivalent to the North Star in the Southern Hemisphere.

  The altitude is the angle an object makes with the horizon  

The celestial horizon, also known as the astronomical horizon or the true horizon, is an imaginary plane that represents the boundary between the Earth and the sky when viewed from a specific location. It is an important concept in astronomy and navigation.

The celestial horizon is a horizontal plane that extends infinitely in all directions from the observer's position on Earth. It divides the celestial sphere into two parts: the visible sky above the horizon and the invisible sky below the horizon.

From an observer's perspective, the celestial horizon is the apparent boundary where the sky meets the Earth. It is the lowest part of the celestial sphere that is visible, and all celestial objects that are above the horizon can be observed. Objects below the horizon, on the other hand, are obscured from view by the Earth itself.

The position of the celestial horizon depends on the observer's location on Earth. For example, an observer standing on the seashore will have a different celestial horizon compared to someone on top of a mountain or in a flat plain. The altitude and shape of the terrain also affect the apparent position of the horizon.

Astronomers and navigators often use the celestial horizon as a reference point to determine the positions of celestial objects. The altitude of an object above the horizon, along with its azimuth (measured along the horizon), is used to locate and track celestial bodies. By measuring the angles between celestial objects and the horizon, astronomers can calculate their positions and movements.

It's important to note that the celestial horizon is distinct from the geometrical horizon, which refers to the boundary where the Earth's surface and the sky visually appear to meet. The geometrical horizon is affected by factors like atmospheric refraction and the observer's height above sea level, whereas the celestial horizon is an abstract plane used in celestial coordinate systems and calculations.


Altitude and azimuth are two important angular coordinates used in astronomy and navigation to specify the position of celestial objects in the sky.

  1. Altitude: Altitude refers to the angular height of a celestial object above the observer's horizon. It is measured in degrees and ranges from 0° at the horizon to 90° at the zenith (the point directly overhead). A celestial object with an altitude of 0° is on the horizon, while an object with an altitude of 90° is directly overhead.

  2. Azimuth: Azimuth represents the angular distance of a celestial object along the observer's horizon, measured in degrees from a reference point. The reference point is usually defined as 0° azimuth and is commonly chosen as north or south. Azimuth values increase clockwise from the reference point, so east corresponds to 90°, south to 180°, west to 270°, and north (or 360°) to complete the circle.

By using altitude and azimuth together, it is possible to specify the position of a celestial object in the sky relative to the observer's location. For example, an object directly overhead (zenith) would have an altitude of 90° regardless of its azimuth. An object on the horizon directly to the south would have an altitude of 0° and an azimuth of 180°.

These coordinates are crucial for astronomers and navigators to locate and track celestial objects. By measuring the altitude and azimuth of a celestial body at a specific time, observers can determine its position and monitor its movement across the sky. These coordinates are often used in conjunction with other coordinate systems, such as the equatorial coordinate system, to precisely locate and track objects in the sky.


Useful Links