Celestial objects are usually best placed for observation or photography when they are highest in the sky. This is when we are looking through the least amount of the Earth's atmosphere.

Objects are highest in the sky when they are transiting the meridian, an imaginary line that runs from north to south that passes through the zenith.

Deep-sky objects are located by right ascension and declination - a set of celestial coordinates equivalent to longitude and latitude on the Earth, but projected onto the celestial sphere.

Because the Earth rotates, celestial objects seem to move across the sky, so a different hour of right ascension is constantly moving across the meridian as time passes.

Sidereal time is equal to the hour of right ascension that is on the meridian at a given time.

Using the Calculator Based on Observing Time and Date

Here is a quick way to plan an observing or astrophotography session:

• Make sure your computer is set with the correct time, date, and location time zone.

• Click the "Use Computer System Time" button in the Local Sidereal Time Calculator.

• Note the local sidereal time that is displayed.

• If you are not going to observe immediately, add the number of hours until your observing time to the displayed sidereal time.

• Go to the Master Objects List, and scroll down until you find the right ascension that is the same as the sidereal time that has been calculated. These are the objects that will be on the meridian at that time.

For example, lets say it is June 21 and you are located in the eastern time zone of the United States. Daylight savings time is in effect. You are planning your observing session at 10 a.m. EDT and you plan to observe at 10 p.m. EDT that night. EDT has a -4 hour time zone.

Clicking the "Use Computer System Time" button in the calculator, we see that the local sidereal time at 10 a.m. EDT on June 21 is about 3 hours.

You plan on observing at 10 p.m. EDT that night (12 hours later), so add 12 hours to the sidereal time.

• 3 hours sidereal time + 12 hours = 15 hours sidereal time.

Since sidereal time equals the right ascension of the meridian, 15 hours of right ascension will be on the meridian at 10 p.m. EDT on June 21.

Go to the Master Objects List and see what objects are around 15 hours of right ascension. They will be best placed for photography at 10 p.m EDT on June 21. Some objects, particularly those that go high overhead, can easily be shot well before and well after they transit.

Using the Calculator Based on a Particular Object

To determine when a celestial object is on the meridian, we need to know the right ascension of the object and the local sidereal time. Object right ascension can be found in the master list. We then use the Local Sidereal Time Calculator above to find out when that hour of right ascension will be on the meridian for our observing location.

Suppose we want to shoot M8, the Lagoon Nebula. By doing a search in this book, or looking it up in the Master List of Objects, we can find that the right ascension of M8 is 18 hours 04 minutes.

By then using the Local Sidereal Time Calculator for our observing location, we can input the date and time, and see what the local sidereal time will be (which is the same as the right ascension on the meridian).

If we input, for example, 10 p.m. EDT (22:00 hours in 24 hour time with a -4 time zone for daylight savings time) on June 21, we see that the local sidereal time is 14.93 hours, or just about 15 hours local sidereal time.

Now we know that M8's right ascension is about 18 hours, and that 15 hours or right ascension will be on the meridian at 10 p.m. EDT on June 21. This means that M8 will transit the meridian about 3 hours later, which will be about 1 a.m. EDT local time.

Longitude Correction

If you are not located on a standard longitude, such as 0°, 15°, 30°, 60°, 75°, 90°, 105°, 120°, etc., your sidereal time can vary a bit from what is calculated by the Sidereal Time Calculator.

It is simple to correct for this, just put in the longitude of your observing site and hit enter on your keyboard (sorry, this doesn't work in Internet Explorer - try another browser like Firefox or Chrome).

For example, when it is 10 p.m. EDT local time on June 21 in Philadelphia at 75° West longitude, the sidereal time is 14.93 hours on the meridian. But in Atlanta, Georgia, which is located at 84.39° West longitude, at the same time it is only 14.3 hours of sidereal time. This is because Atlanta is west of Philadelphia, and even though it is in the same eastern time zone, it is somewhat more than halfway to the next time zone, so the sidereal time is a little more than 1/2 hour earlier.

Calculator Notes

The calculator uses 24-hour time:

• Midnight = 0 hours
• Noon = 12 hours

Add 12 to your local time if it is after noon:

• Example: 4 p.m. = (4 + 12) = 16 hours in 24-hour time.

The calculator also uses decimal hours:

• 12.00 hours = 12 hours 00 minutes
• 12.25 hours = 12 hours 15 minutes
• 12.50 hours = 12 hours 30 minutes
• 12.75 hours = 12 hours 45 minutes

You can input a different time, date and longitude into the calculator and hit the enter key on your computer's keyboard. Again, this doesn't work in Internet Explorer, so try another browser.

Note that the East / West drop down selection box for the hemisphere doesn't work as you would expect. While it appears to let you change its value, it doesn't change the sidereal time. It is actually more of an information display than a selection function. The hemisphere is correctly displayed if you have the correct location set for your computer, and the sidereal time displayed is correct. Changing the longitude of your observing site does correctly change the sidereal time.

Time zones are calculated from the prime meridian in Greenwich, England which is located at zero degrees longitude. Time zones are usually one hour apart and are spaced about every 15 degrees of longitude. This is because there are 360 degrees of longitude around the Earth and 24 hours in a day (360/24 = 15). Time zones west of Greenwich use negative numbers.

• Example: When it is noon in Greenwich, the Sun is just about rising in New Orleans in the United States. New Orleans is west of Greenwich at 90 degrees longitude, and so it is 6 hours earlier in New Orleans, which is in the -6 time zone. Your time zone can be easily located at different sites on the internet such as this one. Just do a search for "time zones" in Google or Bing or DuckDuckGo

Don't forget about daylight saving time if you manually enter a value for the time zone:

• Example: eastern standard time is normally -5 hours, but when daylight saving time is in effect, it is -4 hours.

Geographic locations on the Earth that are west of Greenwich are designated with a negative longitude, up to the International Date Line, which is the imaginary line opposite the prime meridian on the Earth. North and South America, or example, are negative (west) longitude. Asia, Australia and most of Europe and Africa are positive (east) longitude.

Thanks to Rekhesh Mohan for the code for this local sidereal time calculator.

Sidereal Time

Sidereal time is based on the rotation of the Earth in relation to the stars.

Our normal clocks keep time in relation to the position of the Sun. This is called "solar time."

It takes about 24 hours for the Sun to come back to our local meridian. 24 hours is what we consider to be a "solar day."

But the Earth actually rotates once on its own axis in 23 hours, 56 minutes, 4.09 seconds in relation to the stars, a little bit less than the 24 hours of a solar day. This is called a "sidereal day."

The reason a solar day is longer than a sidereal day is because as the Earth is rotating on its own axis, it is also revolving around the Sun. One day later, the Earth has moved in its orbit, and therefore the Sun won't be exactly in the same place in the sky one axial rotation later. It will take about an extra four minutes (3 minute and 53.91 seconds) for the Sun to get back to the meridian.

The stars in the night sky, however, are so far away that they appear fixed as the Earth rotates on its own axis as it revolves around the Sun. Therefore, time kept by the stars will differ from time kept by the Sun.

To keep track of where the stars and deep-sky objects are, we use sidereal time which equals the right ascension that is on the meridian at a given time.