Ephemeris

Ephemeris

Introduction

This is an online general purpose ephemeris, a table that gives the positions of celestial objects. It can display a range of information for the planets, the major stars and the Messier objects as viewed from any location on Earth.

I designed it mainly for naked-eye or binocular work. It is especially useful for situations like, "What is that bright object about 40 degrees up in the East".

Features

• You can use your local time or UTC (GMT), whichever you prefer, or you can just click on the Now box.
• You can use the handy list of timezones to enter your timezone, which is needed if you are using your local time.
• You can choose your location from a list of provided cities, or you can enter your latitude and longitude in a variety of formats.
• You can fine tune things with the local temperature, pressure and altitude, but you don't have to.
• You get the seasons and the moon events.
• You get all the naked-eye visible planets.
• You can also select a range of common stars and messier objects.
• Hint:To see if a constellation is up, select all stars, and choose "Only above the horizon". Then sort list of stars by constellation.
• You can do some filtering to exclude irrelevant objects.
• You can save your settings for next time.
• Two Submit buttons, one top and one bottom, for when you are just changing the date and don't want to scroll down to the bottom button.

Usage

• Select the time and date, or choose Now. Initially, it is set to current time in UTC.
• Select UTC or local time and your timezone.
• Select your location:
• a city from the drop-down list, or
• enter the latitude and longitude.
• Enter the temperature, elevation, and barometric pressure of your location, or leave them blank to use the defaults.
• These affect the angles only slightly except for refraction when the object is very close to the horizon. You probably don't need to bother most of the time.
• You do need to bother if you want really accurate rise and set times, since refraction does affect those. The difference between using the default pressure and actual pressure can be a minute or two.
• You can disable refraction completely by setting pressure to 0.
• Cities come with a default elevation (which is displayed in the output). You don't need to set elevation if you chose a city.
• Barometric pressure is the sea level equivalent, i.e. the one that the TV and newspapers report.
• Magnitude: magnitude is a measure of brightness (or dimness, if you prefer); the dimmer the object the higher the number. The scale ranges from negative (very bright objects) to positive (dimmer objects). Most visible stars are between roughly 1—6.
• Typically, with the naked eye you can see up to about magnitude 3 objects in an urban area, and up to about 6 in a rural area. Binoculars increase that quite a bit, up to about 9 in a rural area.
• Select which of the planets you wish to see (temporarily disabled; you get the lot, free!).
• Select none or more stars you wish to see (use the control key to select multiple stars; ctrl-a to select all of them).
• Do the same with the Messier objects.
• Choose whether to display the results in Altitude/Azimuth format, or in Right Angle/Declination format.
• Choose to display only those objects that are above the horizon.
• Choose to display only those objects brighter than a certain value (the lower the value the more objects are filtered out; higher values will include dimmer objects).
• Example: display only those objects brighter than mag 4 since you live in a city and cannot see dimmer objects anyway. Enter 4 in the box. The results will display everthing from magnitude -99 to 4 and will exclude everything from 4 to 99.
• Choose to save settings, which stores your date, timezone and location information for later so you won't have to re-enter them again. It uses cookies so it won't work if you have cookies disabled for this site.
• Clear settings removes any settings you have previously stored, e.g. you want to remove a saved city. (It may not take effect until a browser restart.)

Output

• You can sort the columns by clicking on the column heading. Click again to reverse the sort.
• The rise and set times are always the next event in the immediate future. This can be confusing if you are comparing times with other websites like heavens-above.com that provide you with a previous and a next event. This ephemeris always looks forward. If you want yesterday's times, change the date.
  Set time may occur before rise time, or vice versa. When a body is currently above the horizon, it sets before it rises next. When the body is below the horizon, e.g. on the other side of the Earth, it rises before it sets. You can tell which event occurs first by checking if the body is above or below the horizon currently.
• The crescent moon is the time of sunset immediately after the new moon (whose time may have been during the day), once the moon sets after the sun and it is at least 10° above the horizon (10° chosen arbitrarily–sometimes the crescent moon is only a couple of degrees above the horizon and you can't really see it). This may be the day of the new moon, or it may be a day or two later. I included this for interest's sake as the new crescent moon is quite pretty. It is not supposed to be accurate enough for religious observations; it does not take the height of the your horizon into account.
• Some stars either never set or never rise for your location, which means there is no set time or rise time. In that case, the time is shown as -1 instead.
• TransAlt, transit altitude, is the altitude at transit time, which is the time an object is the highest in the sky (i.e. passes through North).
• Local noon is the time that the Sun is highest in the sky (the Sun's transit time). Locations in the east of a timezone have their local noon earlier than locations in the west.

Restrictions

• Date restrictions are 1 A.D. — 9999 A.D., due to Python's date module which cannot go further back than 1 A.D. The astronomical library I use can go further back, but I haven't implemented that feature because it would be a lot of work for the very few times it would be used. Email me if this is an issue for you and I might change my mind.
• For years earlier than 1900, the local time and UTC time output is not formatted nicely due to a restriction in the strftime function in Python. This may result in odd displays such as "12:32:59.9999" instead of "12:33:00".
• I use cookies to store your location and preferences, and javascript to automatically blank some input when you select a conflicting choice, like blanking latitude and longitude when you select a city. If you delete cookies and disable javascript, you will disable those useful features and might get inaccurate results.
  • You can view the javascript in the page's source if you want to check it out.
• Latitudes higher than 67° or lower than -67° may result in a Python error. This is because sometimes the Sun or Moon either never set or never rise at those latitudes. I need to add the ability to handle those situations smoothly.

About

This is version 1.1, usable but not tidy. It is written in python using the pyEphem module. pyEphem uses the astro library from xephem, the well known Unix astronomy application.

pyEphem made the astronomy calculations easy. Almost all the development work was in data validation, getting cookies to work properly, and the html.

Tested on google-chrome, opera and firefox. Not yet tested on IE.

To Do

• Handle high and low latitudes.
• Add options to allow different fields in the output: next/prev rise/alt; transit time/alt.
• create star charts. I think I know how to do it and am now mulling over which projection method to use. I'm leaning towards stereographic at the moment.
• Add ability to display satellite passes: ISS and Hubble.

Credit

The telescope icon is courtesy of the Oxygen Icon Team under the Creative Common Licence.

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