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0001 <sect1 id="tool-altvstime">
0002 <title>Altitude vs. Time Tool</title>
0003 <indexterm><primary>Tools</primary>
0004 <secondary>Altitude vs. Time Tool</secondary>
0005 </indexterm>
0007 <screenshot>
0008 <screeninfo>
0009 The Altitude vs. Time Tool
0010 </screeninfo>
0011 <mediaobject>
0012   <imageobject>
0013     <imagedata fileref="altvstime.png" format="PNG"/>
0014   </imageobject>
0015   <textobject>
0016     <phrase>Altitude vs. Time Plotter</phrase>
0017   </textobject>
0018 </mediaobject>
0019 </screenshot>
0021 <para>
0022 This tool plots the altitude of any objects as a function of time,
0023 for any date and location on Earth.  The top section is a graphical
0024 plot of altitude angle on the vertical axis, and time on the horizontal
0025 axis.  The time is shown both as standard local time along the bottom,
0026 and <link linkend="ai-sidereal">sidereal time</link> along the top.
0027 The bottom half of the graph is shaded green to indicate that points
0028 in this region are below the horizon.
0029 </para>
0030 <para>
0031 There are a few ways to add curves to the plot.  The simplest way to
0032 add the curve of an existing object is to simply type its name in the
0033 <guilabel>Name:</guilabel> input field, and press Enter, or the
0034 <guibutton>Plot</guibutton> button.  If the text you enter is found in
0035 the object database, the object's curve is added to the graph.  You
0036 can also press the <guibutton>Find Object...</guibutton> button to open the
0037 <link linkend="findobjects">Find Object Window</link> to select an
0038 object from the list of known objects.  If you want to add a point
0039 that does not exist in the object database, simply enter a name for
0040 the point, and then fill in the coordinates in the
0041 <guilabel>RA:</guilabel> and <guilabel>Dec:</guilabel> input fields.
0042 Then press the <guibutton>Plot</guibutton> button to add the curve for
0043 your custom object to the plot (note that you have to pick a name that
0044 does not already exist in the object database for this to work).
0045 </para>
0046 <para>
0047 When you add an object to the plot, its altitude vs. time curve is
0048 plotted with a thick white line, and its name is added to the listbox
0049 at the lower right.  Any objects that were already present are plotted
0050 with a thinner red curve.  You can choose which object is plotted with
0051 the thick white curve by highlighting its name in the listbox.
0052 </para>
0053 <para>
0054 These curves show the objects' Altitude (angle above the <link
0055 linkend="ai-horizon">horizon</link>) as a function of time.  When
0056 a curve passes from the lower half to the upper half, the object has
0057 risen; when it falls back to the lower half, it has set.  For example,
0058 in the screenshot, the minor planet <firstterm>Quaoar</firstterm> is
0059 setting at around 15:00 local time, and is rising at about 04:00 local
0060 time.
0061 </para>
0062 <para>
0063 The plot can be easily zoomed and panned using the mouse. The zooming level
0064 can be adjusted using the mouse scroll <mousebutton>wheel</mousebutton>. When the plot is zoomed out,
0065 the zooming level will be automatically limited to a default minimum value,
0066 such as the axes’ ranges are able to cover the entire plot (i.e. on vertical
0067 axis, the maximum altitude value should touch the top edge of the plot frame).
0068 The same principle is applied when the plot is panned using the mouse movement.
0069 For panning the plot, all you need to do is to click on the point of the plot
0070 you want to pan and then drag the mouse on your desired direction: left, right,
0071 up or down.
0072 </para>
0073 <para>
0074 Another useful feature that Altvstime tool provides is its ability to compute
0075 and mark the Rise/Set/Transit times. This can be accomplished by a simple press
0076 of one of the <guibutton>Rise</guibutton>, <guibutton>Set</guibutton> or
0077 <guibutton>Transit</guibutton> buttons. When one of these buttons is pressed,
0078 a filled colored circle will be marked on the curve. The color used for Rise,
0079 Set and Transit dots are: red for Rise, blue for Set and green for Transit time.
0080 For a more intuitive way of using these buttons, they are marked with the color
0081 of their specific dots. This way, the user can easily understand the plot and make
0082 the association between a certain color and its significance.
0083 </para>
0084 <para>
0085 Altvstime tool provides an easy way to find out the altitude of a sky-object at
0086 a certain moment of time using the <guibutton>Compute</guibutton> button. After
0087 you select the object from the Objects list and set the moment of time, the
0088 altitude can be computed by pressing the <guibutton>Compute</guibutton> button.
0089 But this is not the only way you can find out the altitude. Another method is to
0090 hover with mouse above one curve and thus, a tooltip containing the Local Time,
0091 Local Sidereal Time and Altitude will come up. This way, you can read the
0092 information in real time.
0093 </para>
0094 <para>
0095 The Altitude of an object depends on both where you are on Earth, and
0096 on the Date.  By default, the Tool adopts the Location and Date from the
0097 current &kstars; settings.  You can change these parameters in the
0098 <guilabel>Date &amp; Location</guilabel> Tab.  To change the Location,
0099 you can press the <guibutton>Choose City...</guibutton> button to open
0100 the <link linkend="setgeo">Set Geographic Location</link> Window, or
0101 enter Longitude and Latitude values manually in the input fields, and
0102 press the <guibutton>Update</guibutton> button.  To change the Date,
0103 use the spin and combo boxes, then press
0104 <guibutton>Update</guibutton>.  Note that any curves you had already
0105 plotted will be automatically updated when you change the Date and/or
0106 Location.
0107 </para>
0109 <tip>
0110 <para>Exercise:</para>
0111 <para>
0112 Plot the Sun's Altitude curve.  Make sure the geographic location is not
0113 near the equator.  Change the Date to some time in June, and then again to
0114 sometime in January.  You can see easily why we have seasons; in the
0115 winter, the Sun is above the horizon for less time (the days are shorter),
0116 and its altitude is never very high.
0117 </para>
0118 </tip>
0121 </sect1>