diff --git a/app/src/main/java/com/forrestguice/suntimeswidget/settings/HelpPreference.java b/app/src/main/java/com/forrestguice/suntimeswidget/settings/HelpPreference.java
new file mode 100644
index 000000000..083b11624
--- /dev/null
+++ b/app/src/main/java/com/forrestguice/suntimeswidget/settings/HelpPreference.java
@@ -0,0 +1,155 @@
+/**
+    Copyright (C) 2022 Forrest Guice
+    This file is part of SuntimesWidget.
+
+    SuntimesWidget is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    SuntimesWidget is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with SuntimesWidget.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package com.forrestguice.suntimeswidget.settings;
+
+import android.annotation.TargetApi;
+import android.content.Context;
+import android.content.Intent;
+import android.content.res.TypedArray;
+import android.net.Uri;
+import android.preference.DialogPreference;
+import android.support.annotation.Nullable;
+import android.util.AttributeSet;
+import android.util.TypedValue;
+import android.view.Gravity;
+import android.view.View;
+import android.view.ViewGroup;
+import android.widget.LinearLayout;
+import android.widget.TextView;
+
+import com.forrestguice.suntimeswidget.R;
+import com.forrestguice.suntimeswidget.SuntimesUtils;
+
+/**
+ * A "preference" (non-persistent) that displays help content in a dialog.
+ */
+@TargetApi(11)
+public class HelpPreference extends DialogPreference
+{
+    private CharSequence helpText = "";
+    private String helpLink = null;
+    private TextView helpView;
+
+    @TargetApi(21)
+    public HelpPreference(Context context) {
+        super(context);
+    }
+
+    public HelpPreference(Context context, AttributeSet attrs) {
+        super(context, attrs);
+        setHelpText(context, attrs);
+    }
+
+    @TargetApi(21)
+    public HelpPreference(Context context, AttributeSet attrs, int defStyleAttr)
+    {
+        super(context, attrs, defStyleAttr);
+        setHelpText(context, attrs);
+    }
+
+    @TargetApi(21)
+    public HelpPreference(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)
+    {
+        super(context, attrs, defStyleAttr, defStyleRes);
+        setHelpText(context, attrs);
+    }
+
+    @Override
+    protected View onCreateDialogView()
+    {
+        Context context = getContext();
+
+        float marginTopBottom = TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 8, getContext().getResources().getDisplayMetrics());
+        float marginLeftRight;
+        TypedArray a = context.obtainStyledAttributes(new int[] { R.attr.dialogPreferredPadding });
+        marginLeftRight = context.getResources().getDimension(a.getResourceId(0, R.dimen.settingsGroup_margin));
+        a.recycle();
+
+        helpView = new TextView(getContext());
+        LinearLayout.LayoutParams params1 = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
+        params1.gravity = Gravity.START;
+        params1.setMargins((int)marginLeftRight, (int)marginTopBottom, (int)marginLeftRight, (int)marginTopBottom);
+        helpView.setLayoutParams(params1);
+
+        LinearLayout dialogView = new LinearLayout(getContext());
+        dialogView.setOrientation(LinearLayout.VERTICAL);
+
+        LinearLayout.LayoutParams layoutParams = new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
+        layoutParams.gravity = Gravity.CENTER;
+        dialogView.setLayoutParams(layoutParams);
+
+        dialogView.addView(helpView);
+        return dialogView;
+    }
+
+    @Override
+    protected void onBindDialogView(View v)
+    {
+        super.onBindDialogView(v);
+        helpView.setText(helpText);
+    }
+
+    @Override
+    protected void onDialogClosed(boolean result)
+    {
+        if (result && (getPositiveButtonText() != null && helpLink != null)) {
+            openHelpLink(getContext());
+        }
+    }
+
+    @Override
+    protected Object onGetDefaultValue(TypedArray a, int i) {
+        return a.getInt(i, 0);
+    }
+
+    @Override
+    protected void onSetInitialValue(boolean restoreValue, Object defaultValue) {
+    }
+
+    public void setHelpText(Context context, @Nullable AttributeSet attrs)
+    {
+        if (attrs != null)
+        {
+            String buttonText = null;
+            TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.HelpPreference, 0, 0);
+            try {
+                this.helpText = SuntimesUtils.fromHtml(a.getString(R.styleable.HelpPreference_helpText));
+                helpLink = a.getString(R.styleable.HelpPreference_helpLink);
+                buttonText = a.getString(R.styleable.HelpPreference_moreHelpButtonText);
+            } finally {
+                a.recycle();
+            }
+            setPositiveButtonText(buttonText);
+        }
+    }
+    public CharSequence getHelpText() {
+        return helpText;
+    }
+
+    public void openHelpLink(Context context)
+    {
+        if (context != null && helpLink != null) {
+            context.startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse(helpLink)));
+        }
+    }
+    public String getHelpLink() {
+        return helpLink;
+    }
+
+}
diff --git a/app/src/main/res/values/attrs.xml b/app/src/main/res/values/attrs.xml
index 3c43a05e0..901259aea 100644
--- a/app/src/main/res/values/attrs.xml
+++ b/app/src/main/res/values/attrs.xml
@@ -171,4 +171,10 @@
         <attr name="zeroValueText" format="string" />
     </declare-styleable>
 
+    <declare-styleable name="HelpPreference">
+        <attr name="helpText" format="string" />
+        <attr name="helpLink" format="string" />
+        <attr name="moreHelpButtonText" format="string" />
+    </declare-styleable>
+
 </resources>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 1535c4380..41fdf7128 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -27,6 +27,7 @@
     <string name="configAction_worldMap">World Map</string>
     <string name="configAction_moon">Moon</string>
     <string name="configAction_help">Help</string>
+    <string name="configAction_onlineHelp">Online Help</string>   <!-- TODO -->
     <string name="configAction_overflow">More</string>
     <string name="configAction_settings">Settings</string>
     <string name="configAction_aboutWidget">About</string>
@@ -289,6 +290,7 @@
     <string name="configLabel_general_calculator">Data Source:</string>         <!-- spinner label -->
     <string name="configLabel_general_calculator_sun">Sun Data Source</string>         <!-- preference label -->
     <string name="configLabel_general_calculator_moon">Moon Data Source</string>         <!-- preference label -->
+    <string name="configLabel_general_calculator_help">About Data Sources</string>    <!-- preference title / help button text --> <!-- TODO -->
 
     <string name="configLabel_general_timeFormatMode">Time Format:</string>         <!-- spinner label -->
 
@@ -1870,6 +1872,18 @@
     <string name="calculator_displayString_time4a_noaa">Follows algorithms published by the National Oceanic and Atmospheric Administration.</string>
     <string name="calculator_displayString_time4a_cc">Follows algorithms published by Dershowitz/Reingold in their book "Calendrical Calculations" (third edition).</string>
     <string name="calculator_displayString_time4a_4j">Based on astronomical calculations published by Jean Meeus in his book "Astronomical Algorithms" (second edition).</string>
+    <string name="help_datasources">
+        <![CDATA[
+        Suntimes uses third-party libraries to perform astronomical calculations.<br/>
+        <br/>
+        The <b>data source</b> setting allows choosing between different libraries (or choosing between different algorithms offered by those libraries). The default is <b>time4a-time4j</b>.<br/>
+        <br/>
+        This setting affects the speed and accuracy of calculations, and may limit which features are available. <br/>
+        <br/>
+        For more information about available choices, or extending the app to support other libraries, see the online help.
+    ]]>
+    </string>   <!-- TODO -->
+    <string name="help_datasources_url">https://github.com/forrestguice/SuntimesWidget/wiki/Help</string>
 
     <!-- pref defaults -->
     <string name="def_appwidget_0_general_calculator" translatable="false">time4a-time4j</string>
diff --git a/app/src/main/res/xml/preference_general.xml b/app/src/main/res/xml/preference_general.xml
index 725589715..a8cc914e9 100644
--- a/app/src/main/res/xml/preference_general.xml
+++ b/app/src/main/res/xml/preference_general.xml
@@ -57,6 +57,17 @@
             android:dialogTitle="@string/configLabel_general_calculator_moon"
             android:summary="%s"
             android:entries="@null" android:entryValues="@null" android:defaultValue="@string/def_appwidget_0_general_calculator_moon" />
+
+        <com.forrestguice.suntimeswidget.settings.HelpPreference
+            android:key="appwidget_0_general_calculator_help"
+            android:persistent="false"
+            android:title="@string/configLabel_general_calculator_help"
+            android:icon="?attr/icActionHelp"
+            app:helpText="@string/help_datasources"
+            app:helpLink="@string/help_datasources_url"
+            app:moreHelpButtonText="@string/configAction_onlineHelp"
+            />
+
     </PreferenceCategory>
 
     <!--<PreferenceCategory