"Working with time"

20.0 Time Panel You can use RedShift to simulate astronomical events that took place in the past, or are forecast to take place at some time in the future. You can enter a new time and date using the Time panel. Local time on Earth depends on the longitude of your current location, so astronomers normally specify the time of an event using Universal Time. Universal Time is the current time at the prime meridian (0 degrees longitude) without daylight saving applied. If you have chosen a location away from the Earth's surface you may only specify time as Universal Time. Each time you run RedShift it calculates the current Universal Time according to the time and date set on your computer (system time). Refer to section 39. Customising RedShift to understand more about how this is done. The correct position of the stars and planets are calculated using your system's time and date so - if you wish to use RedShift's real time feature - it is important that these are correctly set. RedShift also lets you work using the Julian date. The Julian date is the number of days which have elapsed since noon at Greenwich on the 1^st of January 4713 BC. Use the Time pop-up menu in the Time panel to select Local time, Universal time or Julian date format. 20.1 Entering a new date and time RedShift 3 lets you set the date and time anywhere between 4712 BC and 9999 AD. RedShift automatically adjusts the epoch for the time set in the Time panel so as to allow for the effects of precession. You can enter a new date and time by editing the date and time fields in the Date and Time settings boxes. Click a value (such as the day, month or year) and type new values. Use the tab key to step through each of the date and time values.
	Use the Date and Time pop-up menus using the buttons to the right of the date and time settings boxes to select from frequently used settings.
Select European or US date formats using Preferences (File menu, Preferences...). Select 12 hour or 24 hour clock format using Preferences (File menu, Preferences...). 20.2 Adjusting local time zone Universal Time is calculated using the system time and the time zone for your home location set in Preferences. RedShift does this automatically. Local time for the location currently set in RedShift is calculated from Universal Time and the local time zone. The local time zone for the present RedShift location is displayed to the left of the time box (only when local time display is selected). At your home location or within 2 degrees latitude and longitude of it, your home time zone information is used. Refer to section 39. Customising RedShift to see how to change this. Away from your home location, RedShift calculates local time (strictly speaking, civil time) base on longitude, to the nearest hour. By default, no daylight savings adjustments are made. Real civil time zones are not based exactly on longitude, and daylight saving time has many local variations. So if you know the exact time zone and daylight savings conditions for a particular location, you may wish to override this automatic calculation by changing the value in the Time zone field. 1. Open the Time zone dialog from the Time pop-up menu using the button located to the right of the Time settings box. 2. Enter the number of hours difference between local time at your currently set location, and Universal time (GMT). This will change local time, while keeping Universal time constant. 3. To apply daylight saving time corrections, check the daylight saving time checkbox and enter the starting and ending dates. IMPORTANT NOTE: this dialog applies only to the current RedShift location, and this information will be lost when you change to a new location. These changes do not affect the information for your home location, which is set in the Preferences dialog. 20.3 Control Time panel Astronomical events can take hours, days or even years to occur. To simulate astronomical events, you can speed up time using the Control time panel. Use the Time Steps pop-up menu to set the units using the button located to the right of the Step settings box. Select units of minutes, hours, days or years. Edit the current step value in the Step field (click the current value to select it then enter a new value). This allows you to control the time step RedShift uses each time it updates the screen (for example you might choose time steps of one day to watch the Moon orbit the Earth). Click the forward or back arrows in the Control Time panel to run time forwards or backwards in current time steps (time tracking). You can also start time running by selecting Run Timer from the Controls menu. If you have the Time panel open while running time you can see that the current time is being updated with each time step. Click the Stop button in the Control time panel to stop time running. Use the buttons either side of the stop button to step forward or back one time step at a time. After running time you can return to the time from which you started by clicking the button in the top right corner of the Control Time panel. The actual speed at which RedShift can move the objects on the screen depends on the speed of your processor and the display filter settings. Remember that RedShift accurately calculates the position of a planet or moon for each new time step; it also calculates where the Sun currently is in relation to each planet and moon in order to show the correct phase (shadow) on the surface of the planet or moon. 20.4 Calculating real time RedShift can calculate the actual motion of planets and other objects as each minute passes. Start real time by choosing Real time from the Controls menu. You can then choose whether to start from RedShift's time setting, or to follow the exact time from your computer's clock. Every minute, RedShift updates the Sky window to the new time. Stop real time tracking by choosing Stop timer from the control menu, or by clicking the stop button in the Control Time panel. If you run time from the Control Time panel, real time tracking stops automatically.