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<sect1 id="tool-altvstime">
<title>Altitude vs. Time Tool</title>
<indexterm><primary>Tools</primary>
<secondary>Altitude vs. Time Tool</secondary>
</indexterm>
<screenshot>
<screeninfo>
The Altitude vs. Time Tool
</screeninfo>
<mediaobject>
<imageobject>
<imagedata fileref="altvstime.png" format="PNG"/>
</imageobject>
<textobject>
<phrase>Altitude vs. Time Plotter</phrase>
</textobject>
</mediaobject>
</screenshot>
<para>
This tool plots the altitude of any objects as a function of time,
for any date and location on Earth. The top section is a graphical
plot of altitude angle on the vertical axis, and time on the horizontal
axis. The time is shown both as standard local time along the bottom,
and <link linkend="ai-sidereal">sidereal time</link> along the top.
The bottom half of the graph is shaded green to indicate that points
in this region are below the horizon.
</para>
<para>
There are a few ways to add curves to the plot. The simplest way to
add the curve of an existing object is to simply type its name in the
<guilabel>Name</guilabel> input field, and press Enter, or the
<guibutton>Plot</guibutton> button. If the text you enter is found in
the object database, the object's curve is added to the graph. You
can also press the <guibutton>Browse</guibutton> button to open the
<link linkend="findobjects">Find Object Window</link> to select an
object from the list of known objects. If you want to add a point
that does not exist in the object database, simply enter a name for
the point, and then fill in the coordinates in the
<guilabel>RA</guilabel> and <guilabel>Dec</guilabel> input fields.
Then press the <guibutton>Plot</guibutton> button to add the curve for
your custom object to the plot (note that you have to pick a name that
does not already exist in the object database for this to work).
</para>
<para>
When you add an object to the plot, its altitude vs. time curve is
plotted with a thick white line, and its name is added to the listbox
at the lower right. Any objects that were already present are plotted
with a thinner red curve. You can choose which object is plotted with
the thick white curve by highlighting its name in the listbox.
</para>
<para>
These curves show the objects' Altitude (angle above the <link
linkend="ai-horizon">horizon</link>) as a function of time. When
a curve passes from the lower half to the upper half, the object has
risen; when it falls back to the lower half, it has set. For example,
in the screenshot, the minor planet <firstterm>Quaoar</firstterm> is
setting at around 15:00 local time, and is rising at about 04:00 local
time.
</para>
<para>
The Altitude of an object depends on both where you are on Earth, and
on the Date. By default, the Tool adopts the Location and Date from the
current KStars settings. You can change these parameters in the
<guilabel>Date & Location</guilabel> Tab. To change the Location,
you can press the <guibutton>Choose City...</guibutton> button to open
the <link linkend="setgeo">Set Geographic Location</link> Window, or
enter Longitude and Latitude values manually in the input fields, and
press the <guibutton>Update</guibutton> button. To change the Date,
use the <guilabel>Date</guilabel> picker widget, then press
<guibutton>Update</guibutton>. Note that any curves you had already
plotted will be automatically updated when you change the Date and/or
Location.
</para>
<tip>
<para>Exercise:</para>
<para>
Plot the Sun's Altitude curve. Make sure the geographic location is not
near the equator. Change the Date to some time in June, and then again to
sometime in January. You can see easily why we have seasons; in the
winter, the Sun is above the horizon for less time (the days are shorter),
and its altitude is never very high.
</para>
</tip>
</sect1>
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