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