Finding the Planets in Your Local Night Sky
Where in the night sky should I look for a planet tonight? In which direction and how high up will it be?
The location of a planet (or any other celestial body) in your local night sky depends upon several factors: the constellation in which it is positioned, your geographical latitude and longitude and the date and time at which you observe. To find a planet in the night sky at any particular date and time, we must know two things: a direction in which to look along the observer's horizon (eg. Southeast, East-Southeast) and an angle to look above the horizon (known as altitude or elevation).
To activate the program, click on the button below (the program will open in a pop-up window).
The default location is New York, USA. To select your own location and then find a planet, refer to the 'Finding the Planets ..' section below.
An animated tutorial showing how to locate a planet in the night sky using Mini-AstroViewer® can be seen here (783 KB).
Finding the Planets in Your Own
Night Sky using Mini-AstroViewer®
To set your own location, click on the 'Location' button and click on your approximate position on the pop-up world map. If you know your precise latitude and longitude, you can refine your position by pressing the left/right and up/down arrows to move the cross-hair in 1° increments (to find your latitude and longitude, visit the Heavens Above website and enter the name of your nearest town or city in the search box). Having plotted your geographical co-ordinates, click 'OK' and the night sky over your own location will appear in the window, valid for the current time, which is displayed in UT (Universal Time, equivalent to GMT). The applet will initialise displaying the current UT time according to your browser's clock and Time Zone settings (if you would prefer to have the Local Time displayed, use the fully-functional version of the program at Astroviewer.com).
The red circle represents the horizon around you; the lower half of the display represents the part of the sky you are facing. The centre of the circle is the point directly above your head (known as the zenith). The ecliptic (the path along which the Sun, Moon and planets will be found) is marked by a red dashed line, passing as it does through the zodiac constellations. The blue dashed line marks the apparent position of the celestial equator, which arcs across the sky from the due East point on the horizon to the due West point. The program plots stars down to magnitude +5.0.
Sun, Moon and planet colours as they appear in AstroViewer®. The program displays the Moon in its correct phase and position.
The bottom scroll bar rotates the horizon view, allowing for a view in any compass direction; the left-hand scroll bar zooms the sky in or out, and the right-hand scroll bar pans up (to the zenith) or down (to the horizon) whenever the view has been zoomed.
Infomation on a celestial body can be viewed by clicking on the object (in the case of a planet, its magnitude, distance, elongation and apparent diameter). Note that if a planet's elongation (its angular distance from the Sun as seen from the Earth) is less than about 15°, it will not be visible because it is too near the Sun (the much fainter planets Uranus and Neptune are not visible at elongations less than about 20°). Remember that local twilight can affect the visibility of a particular planet, making observation difficult or even impossible. This particularly applies throughout the local summer months at higher latitudes.
To find the location of a planet in your Night Sky, first see if it is above the horizon at the time you are requesting. If it is visible within the circle, move the bottom scroll bar left or or right to rotate the image until the planet is positioned on the vertical red line (the altitude scale). Zoom in to the area using the left-hand scroll bar where necessary.
The direction of the planet at the requested time will be indicated at the bottom (W, SW, etc). The altitude of the planet (its angle above the horizon) can be read off on the altitude scale (it is marked at 10° intervals). Hence if it is three notches up, its altitude is 30° at the displayed time (to understand how to determine a planet's altitude in the night sky, refer to the two diagrams below). If the planet's altitude is less than about 10° it may be difficult to see because of the dimming effect of the Earth's atmosphere and, in town and city locations, the effects of light pollution or skyglow.
Angular altitude (or elevation) is measured as 0º at the horizon (when an object is at the point of rising or setting), 45º when 'half way up the sky' and 90º when directly above the observer's head (at the zenith). In the above picture, the bright star has an an altitude of about 60º (i.e. it is "60º high"). Click to see full-size image (14 KB).
If the planet is not shown within the circle, it is below the horizon and you will have to wait until after it next rises before you can see it (provided it is not too near the Sun). To find when it next rises, click the 'hours forward' button () repeatedly until the planet appears over the eastern horizon, then note down the time and direction this occurs. By clicking the 'minutes/hours forward' buttons (), the planet can then be tracked across the sky for the remainder of the night (using direction and altitude) as described above.
If the planet rises in daylight (i.e. if the Sun is already above the horizon), you will have to wait until after dusk to see it - in which case, 'fast forward' to a time shortly after sunset, then note down the time and direction.
Dawn and Dusk planets
Which planets are visible at dawn and dusk today?
This is easily found from Mini-AstroViewer® by positioning the Sun on the eastern horizon (i.e. sunrise) for dawn planets and on the western horizon (sunset) for dusk planets. Click the 'minutes/hours backward' () or forward () buttons until the Sun is positioned on the required horizon (this will also give the time of local sunrise or sunset). Whichever planets are positioned in the eastern half of the sky at sunrise are dawn planets (visible before sunrise) and whichever planets are positioned in the western half of the sky at sunset are dusk planets (visible after sunset).
In some cases it will be found that a planet is rising whilst the Sun is setting - or likewise, a planet is setting whilst the Sun is rising - in which case the planet is at opposition (i.e. it is directly opposite the Sun in the sky and shining at its brightest for the year). This can only happen in the case of superior planets and it is the best time of the year to view them (for more details, see the Planet Movements page).
Additional AstroViewer® Information
Mini-AstroViewer® is a lightweight version of AstroViewer®, an interactive night sky map that helps you find your way in the night sky quickly and easily. Due to its intuitive interface, it is well suited to beginners in astronomy.
The fully functional, free-to-use version can be accessed at the AstroViewer® website. It has additional features such as a Local Time display, a planet visibility chart for any selected location, a 3D Solar System map, the ability to store user-generated world locations, a 'Find Celestial Body' facility, printing and language options and greater flexibility in the night sky display (see details here).
AstroViewer® is produced by Dirk Matussek.
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Copyright © Martin J Powell 2012