Astronomy - Observing The Night Sky

Observing The Night Sky

The Earth rotates once a day and orbits the Sun once each year.

The first motion causes sky objects to move from east to west, and the second causes different constellations to appear in each season’s sky.

Amateur astronomers use star hopping to go from stars and constellations they know to ones they don’t know yet. First, look for noticeable patterns on the sky’s dome.

One very easy pattern to find at this time of year is the constellation Orion the Hunter.

Orion is easy to find because it contains a very noticeable pattern of three medium-bright stars in a short straight row. These stars represent Orion’s Belt.

If you can find Orion, you can use it to star hop to Sirius, the sky’s brightest star, in the constellation CanisMajor.



The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients.

They believed the Earth was motionless and at the center of creation.

The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over the Earth like a great dome.

The celestial dome with its starry decorations had to be a complete celestial sphere, early sky watchers realized, because we never see a bottom rim as the dome tilts and rotates around the Earth once a day.

Part of the celestial sphere is always setting behind the western horizon, while part is always rising in the east. At any time half of the celestial sphere is above the horizon, half below.

Whenever you want to specify a point on the surface of a sphere, you'll probably use spherical coordinates. In the case of Earth, these are named latitude and longitude.

Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves on the inside of the sky sphere.

They are now called, respectively, declination and right ascension.


Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination.

If you stand on the Earth's equator, the celestial equator passes overhead.

Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination +90°.

On Earth, 0° longitude has long been defined as a line engraved on a brass plate set in the floor under a position-measuring telescope at the Old Royal Observatory in Greenwich.

In the sky, 0h ("zero hours") right ascension is defined as where the plane of the Earth's orbit around the Sun (the ecliptic) crosses the celestial equator in Pisces the March equinox . This point is called, for historical reasons, the First Point of Aries.



Right Ascension is measured in hours (h), minutes (m) and seconds (s) and is similar to longitude on Earth. As the Earth rotates, stars appear to rise in the East and set in the West just like the Sun.

Declination is measured in degrees (°), arc-minutes (') and arc-seconds ("), and is similar to latitude on Earth.

There are 60 arc-minutes in a degree and 60 arc-seconds in an arc-minute.

Declination measures how far overhead an object will rise in the sky, and is measured as 0° at the equator, +90° at the North Pole and -90° at the South Pole.


Hand at Full Arms Length

The word "constellation" seems to come from the Late Latin term cōnstellātiō, which can be translated as "set of stars", and came into use in English during the 14th century.

A constellation is a group of stars that are considered to form imaginary outlines or meaningful patterns on the celestial sphere, typically representing animals, mythological people or gods, mythological creatures. The 88 modern constellations are defined regions of the sky together covering the entire celestial sphere.

In 1928, the International Astronomical Union (IAU) ratified and recognized 88 modern constellations, defined regions of the sky together covering the entire celestial sphere. With contiguous boundaries defined by right ascension and declination.

Therefore, any given point in a celestial coordinate system lies in one of the modern constellations.

Locating Stars


A complete twenty-four-hour time cycle is marked on the rim of the overlay. A full twelve months of calendar dates are marked on the rim of the starchart. The window is marked to show the direction of the eastern and western horizons.

The disk and overlay are adjusted so that the observer's local time of day on the overlay corresponds to that day's date on the star chart disc.

The portion of the star chart visible in the window then represents (with a distortion because it is a flat surface representing a spherical volume) the distribution of stars in the sky at that moment for the planisphere's designed location.

Users hold the planisphere above their head with the eastern and western horizons correctly aligned to match the chart to actual star positions.



Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just as you would see with the naked eye, binoculars or a telescope.


Experience Earth and our solar system, the universe and the spacecraft exploring them, with immersive apps for Mac, PC and mobile devices.



WorldWideTelescope is an open source set of applications, data and cloud services, originally created by Microsoft Research but now an open source project hosted on GitHub. The.NET

American Astronomical Society