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100 Starry Night User’s Guide

Time Flow

By default, time in Starry Night advances

at the same rate as real time. If you run

Starry Night for one hour, the time

onscreen will also advance by one hour.

As the time in Starry Night changes, the

screen continuously updates to reflect the

changing sky.

Note: This means that the

rate of time flow

is the same as real time, but the time itself

may not be the same. For example, let’s

say you open Starry Night at 8 p.m. You

then change the time onscreen to 1 p.m.

and run the program for one hour. At the

end of the hour, the actual time will be 9

p.m, but the time that Starry Night shows

will be 2 p.m.

Changing Time Flow Rate: One of the nice

things about Starry Night is that you can

change the rate at which time flows. Just

like nature programs use time-lapse

photography to show processes which take

place too slow to see in real time, you can

speed up or slow down time to get the best

possible views of an astronomical event.

The rate of time flow is

shown immediately to

the right of the date and

time in the toolbar.

Clicking the arrow opens

a pull-down menu that

displays a list of possible

time steps. The steps fall

into two categories:

multiples of real time,

and

discrete time steps.

Time steps that are

multiples of real time

(i.e. 30x) simply

advance the time in Starry Night at a rate

equal to a multiple of the real rate of time

flow. For example, a time step of 30x

would cause the time in Starry Night Pro

to advance at 30 times the real rate of time

flow. Obviously, the larger the multiple,

the faster time would advance onscreen.

Discrete time steps update the time by a

specific increment. For example, a time

step of 3 minutes means that every time

the screen updates, the time in Starry

Night is advanced by 3 minutes. Discrete

time steps are often useful when

simulating astronomical events. For

example, assume you wanted to see how

Jupiter’s position in the sky changes over

the next few months. You could set the

time to sometime in the evening (for

example, 9 p.m.) and then set the time step

to 1 day. Starry Night would run time

forward, showing the sky at 9 p.m. each

night. If you had chosen a multiple of real

time instead of a discrete time step, you

would see an alternating cycle of day and

night, instead of seeing the sky at the same

time each night.

A few of the discrete time steps listed in

the pull-down menu may be unfamiliar to

you:

Sidereal day: This is the time it takes for

the Earth to rotate once on its axis. It is

four minutes shorter than the day we are

familiar with, which is called the solar day.

The days are not exactly the same length

due to the revolution of Earth around the

Sun.

Sidereal month: This is the time it takes

for the Moon to rotate once around Earth,

approximately 27.3 days. This is shorter

than the month we are familiar with (the

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