Your Sky Help:
Horizon View Control Panel

Date and Time

Now

“Now” displays a star map for the viewpoint specified below at the current moment.

Universal time:

This option permits you to enter any Universal (also known as Greenwich Mean) date and time in the format year/month/day hour/minute/second. You can omit the minutes and seconds of the time, or omit the time entirely if you wish to specify 00:00 UTC.

You can enter dates as far back as Julian day 0, January 1, −4712 and as far into the future as you wish. Note that astronomers and historians use different conventions for years before 1 A.D. In history books, the year that preceded 1 A.D. is called 1 B.C., zero not having come into use in European culture at the time. Astronomers denote the year before 1 A.D. as “year 0”. Thus when an astronomer talks about an eclipse having occurred in the year −412, that's the year historians refer to as “413 B.C.”. In converting historical dates to Julian days, Your Sky assumes the canonical date for the adoption of the Gregorian calendar, Friday, October 15th, 1582. Many countries shifted to the Gregorian calendar much later; in Great Britain, not until 1752. When investigating events in history, make sure you express all dates after October 15th, 1582 in the Gregorian calendar.

Julian day:

Astronomers often have to do arithmetic with dates and times. The Gregorian calendar is sufficiently eccentric that answering a question like “What is the date and time 295.03589 days (10 lunar months) from now?” is a nontrivial exercise. To facilitate computation, astronomers employ the Julian day calendar. The Julian day number for a given moment in time is simply the number of days, whole and fractional, elapsed since noon (12:00) Universal time on 1st January −4712; time is expressed as a fraction of a day. This system allows assigning a positive number to the date and time of any observation in recorded history and arbitrarily far into the future, and permits ordinary arithmetic with dates and times without having to worry about B.C. and A.D., Julian and Gregorian calendars, leap years, and all that. Of course, you need to be able to convert back and forth between Julian days and the civil calendar, but that's what computers are for.

To show a star map for a given Julian day, simply check the box and enter the Julian day number in the box to the right, including a fraction where appropriate. (Be sure to remember that Julian days begin at noon—a Julian date representing midnight will end in “.5”.) Click “Update” to show the map for the designated viewpoint at that day.

Viewpoint

Azimuth: Degrees: North Northeast East Southeast South Southwest West Northwest

The azimuth specifies the direction you're looking toward on the horizon. You can set the azimuth either by choosing one of the cardinal points of the compass from the menu above or by entering an azimuth in degrees in the text field to its right. North is azimuth 0, and azimuth increases in a clockwise direction, hence East is azimuth 90, South 180, and West 270.

Field of view:

The field of view gives the scale of the image—it will span the given number of degrees in altitude and azimuth. The default of 45° gives a large-scale view, comparable to the unaided eye. Reducing the field of view is equivalent to increasing the magnification of a telescope. Field of view is limited to 90°—beyond that distortion of the image becomes unacceptable for horizon views; for an all-sky map, visit the Your Sky Sky Map page.

Observing Site

Obviously, what you see in the sky depends on where you're standing on the Earth. Your Sky, therefore, needs to know the latitude and longitude of your observing site (or other location) in order to correctly display the view from there. Enter the latitude and longitude in the boxes below (the values in the boxes are those of Boston, on the East coast of North America). For each value, enter the degrees and, optionally, minutes and seconds, and be sure to check the correct North/South and East/West box for each setting. You can enter values using either the ISO degree sign, “°”, (which may not be on your keyboard), or with a “d” denoting degrees; Boston's latitude could have been entered as 42d21m24s.

Latitude: North South
Longitude: East West

If you don't know the latitude and longitude of your observing site, or you wish to display horizon views for various locations on Earth, the link above will take you to a list of various places on Earth, mostly cities, which, when clicked, cause Your Sky to show a star map above that location at the current date and time. From the star map, you can click a link to display horizon views at that location.

Display Options

This box controls whether the celestial coordinate system appears in the map. When checked, the pole is marked with a small light blue cross with arms pointing toward the 0, 6, 12, and 18 hour marks on the equator, the celestial equator is drawn in light blue, labeled at each hour of right ascension, and the ecliptic is drawn in red with labels every 15° of ecliptical longitude.

The celestial poles are the points in the sky at the zenith above the Earth's poles; the celestial equator is the projection of the Earth's equator onto the sky, and the ecliptic is the plane in which the Earth orbits the Sun. The obliquity of the ecliptic changes slowly through time as the Earth's inclination varies, and the points at which the ecliptic crosses the equator (the equinoxes) also shift over time due to precession.

If this box is checked, the Moon and planets will be included in the map at their correct positions for the given time and date. The Moon icon will show the Moon with the correct phase, with the lunar north pole at the top of the icon. A table of planet names and icons is given in an accompanying document.

The technique used to calculate the positions of the planets from Mercury through Neptune is valid only for dates less than A.D. 8000; if you specify a date further into the future, the Moon and planets are not plotted on the map and no ephemeris is shown. Pluto has been observed over an insufficient arc of its orbit to allow accurate computation of its position outside the range of years from 1885 through 2099; Pluto is not plotted for dates outside this interval.

Checking this box causes deep sky objects (galaxies, star clusters, gaseous nebulæ, etc.) brighter (in integrated visual intensity) than the given magnitude to be plotted in the star map, using icons to distinguish the different types of objects.

Here we've made a finder chart to locate M31, the great spiral galaxy in Andromeda, labeling brighter stars with their Bayer and Flamsteed designations.

The human visual system, inherited from hundreds of millions of years of mammalian evolution, excels in finding patterns. Spotting the fearful symmetry of a tiger lurking in the bush quickly enough to run away means you're likely to have more children than folks who lack that talent, so we've all inherited the genes of those with the high-end pattern matching meatware. As a result, we see patterns everywhere, even where no pattern is really there. What child, or adult on a lazy day, hasn't gazed at the clouds boiling up on a hot summer afternoon and seen, in the sky, the fantasy images which inhabit our dreams?

We see patterns in the night sky as well. Every civilisation has grouped the stars into constellations representing key aspects of their culture. The constellations in the northern hemisphere sky recognised by Western cultures are largely inherited from classical Greece—they are the goddesses and gods of Olympus and the heroes of Greek mythology.

Constellations in the southern hemisphere were named innumerable times by the multitude of cultures who observed them since antiquity. European culture was, however, unaware of most of the southern sky prior to the Enlightenment and the ensuing age of exploration, so many of them were named for contemporary wonders such as the microscope (Microscopium), air pump (Antila), and clock (Horologium).

Perhaps if our cultural baggage had been lost at the generation-port, we'd look at the stars and see in them the Five Original Marx Brothers, Lucy in the Sky, and Bart's Skateboard. Your Sky serves up the traditional constellations, offering the following options.

If checked, outlines connecting the stars of constellations will be drawn as grey lines.

If checked, each constellation's name is shown in yellow, centred on the midpoint of the constellation.

If checked, the three letter abbreviation will be shown instead of the constellation's full name.

If checked, the boundaries between adjacent constellations are drawn in green.

The constellation boundaries we use today were adopted in 1930 by the International Astronomical Union, based on a partial set of boundaries compiled in 1877 by B. A. Gould. Gould's original boundaries star maps but, for simplicity, ran purely east-west and north-south in the celestial sphere. But as the Earth's axis precesses with regard to the distant stars, completing a full circle every 25,800 years, equatorial coordinates change with regard to the fixed stars, so the original boundaries, defined in 1877, no longer run parallel to lines of longitude and latitude today. (Precession may seem like a minor effect, significant only in the very long term, but it can creep up on you. In the century and a quarter since 1877, precession has moved Polaris three quarters of a degree closer to the north celestial pole—that's one and a half diameters of the full Moon!)

Consequently, it's necessary to adjust the constellation boundaries to account for precession. The boundaries used by Your Sky have been precessed in a simple fashion to the J2000.0 epoch. Approximating the precise 1875 boundaries based on the best available published data would require plotting more than 13,000 vectors and didn't seem worth it, especially since you could hardly see the difference on an all-sky map.

The following controls permit choosing how many stars will appear in the map and select the annotation which accompanies them. Each includes a limiting magnitude field. The brightness of a star is given by its magnitude, with larger numeric magnitude indicating a dimmer star. The brightest star (excluding the Sun) is Sirius, with a magnitude of −1.46. There are about 20 stars brighter (having a magnitude less than) 1.5, and about 100 stars brighter than magnitude 2.6. The number of stars increases rapidly with the magnitude—there are about a thousand stars brighter than magnitude 4.5, and if you use magnitude 5.5 as the limit for naked eye visibility, you include about 3000 stars. Your Sky's horizon views are based on the Yale Bright Star Catalogue, Fifth Edition, which includes more than 9000 stars brighter than magnitude 6.5.

Only stars brighter than specified by this box will be included in the map. Stars are shown as icons; the larger the icon, the brighter the star; due to resolution limits of computer monitors (and convention in many printed star atlases), stars are “binned” into units of one full magnitude.

The following controls select the annotation which accompanies plotted star icons. Note that the annotation settings in the items which follow apply only to stars which are plotted; regardless of the limiting magnitude for an annotation, it will never appear unless the star it applies to is brighter than the cutoff for the map given above.

Many bright stars are named, for example “Polaris”, “Altair”, and “Zubenelgenubi”, and principal stars of constellations are designated by Greek letters often called “Bayer letters” after Johann Bayer who first identified stars this way in his Uranometria of 1603. Multiple stars may bear the same letter and be distinguished by a numeric subscript. Stars are also identified by “Flamsteed numbers”, which simply number the stars within a constellation in order of right ascension. Stars in southern constellations are frequently identified by roman letters.

If this box is checked, named stars brighter than the given magnitude will show the name to the right of the star icon in the map. If you set the magnitude too high, the map may become cluttered with names and difficult to read.

Checking this box causes the Bayer, Flamsteed, or letter designation to be shown for stars brighter than the given magnitude, plotted to the right of the star unless its name is also shown, in which case the code appears to its left.

You can display magnitudes of stars in a given range by checking the “Show magnitudes” box and entering the maximum and minimum magnitude of stars to be so labeled. Magnitudes are shown beneath the stars, to one decimal place with the decimal point omitted, as is the convention for star charts (decimal points being too easily confused with stars). Labeling stars with magnitudes is handy, for example, when you're observing a variable star and want to identify comparison stars within the field that cover the range of the variable.

Unless you're in the middle of the ocean, the horizon is not just a geometric line; local terrain, buildings, people, and loose cows usually make it more interesting to observe. If the “Terrain” box is checked, Your Sky adds simulated random terrain to the bottom of each horizon view—each new view will contain different terrain (and some may contain none at all; the horizon might be flat in that direction).

The roughness of the terrain is specified by a number in the “Roughness” box; smaller (including negative) numbers generate flatter terrain while larger numbers yield more mountainous topography. Roughness is limited to settings between −100 and 2.

The horizon can further be embellished by sprinkling it with randomly selected scenery: houses, barns, trees, people, and livestock. If you find the scenery distracting, uncheck the box and it will go away when you next press “Update”. Scenery is generated only in conjunction with terrain—if you uncheck the “Terrain” box, the scenery will vanish as well.

This field specifies the size, in pixels, of the horizon view to be generated. Map size is restricted to the range of 100 to 1024 pixels; smaller maps would be useless, and larger images take too long to generate and download.

The text used to label the chart will be scaled by the specified factor, with 1.0 the default. Scale factors less than one reduce the size of the text while those greater than one enlarge it compared to the default size.

You can select one the following colour schemes for the horizon view. Samples of each colour scheme are given below, along with a discussion of the applications to which it is suited.

Colour

The default full colour scheme is easiest to view on computer monitors which support 256 (or more) colours. Colour coding coordinates, constellation boundaries and outlines, annotation, and planet icons makes it easier to distinguish the various objects, especially when the map is crowded with many different items.

Black on white background

This is the traditional choice for printed star atlases. If you're planning to print the chart on a black and white printer, this is usually the best choice. The white background allows you to write on the map, and keeps the background from using up lots of ink or toner in your printer.

White on black background

Many astronomers find charts with white stars and text on a black background easier to read with the dim red flashlights they use at the telescope to preserve night vision. Your Sky will produce horizon views in this form, but if you're planning to print the resulting charts, ponder the consequences for your printer. All of that black has to come from somewhere, and that somewhere is generally your printer's ink or toner cartridge. Making lots of white on black maps can use up cartridges at a prodigious rate compared to printing normal text. Also, documents which are largely black cause problems with some printers; laser printers may print a “shadow” on subsequent pages due to excess toner adhering to the imaging surface, and inkjet printers sometimes splatter ink around when feeding the large amount needed for extended regions of black. Your printer may be immune from these foibles, but if it isn't, don't say I didn't warn you.

Night vision (red) If you're taking your laptop computer into the field to use at the telescope, this option may be just the ticket. All items on the map are displayed in red, which doesn't tend to degrade night vision. When using this option, use the display brightness control to reduce the intensity to the minimum level at which you can still easily read the map. Where possible, other material in the map document is displayed red on black, like this paragraph.

Asteroid and Comet Tracking

Given the orbital elements for an asteroid (minor planet) or comet, Your Sky's view views can calculate the position of that object in the sky for any time within the epoch of validity of the elements and display it in the correct location in the sky. Specifying orbital elements is an advanced topic which is discussed in detail in a separate Asteroid and Comet Tracking document.