<entry id="2229178" stat="A" corpus="jmdict" type="jmdict">
<ent_corp type="jmdict">jmdict</ent_corp>
<ent_seq>1184860</ent_seq>
<k_ele>
<keb>下弦の月</keb>
</k_ele>
<r_ele>
<reb>かげんのつき</reb>
</r_ele>
<sense>
<pos>&exp;</pos>
<pos>&n;</pos>
<xref type="see" seq="2147030">上弦の月</xref>
<gloss>last quarter moon</gloss>
<gloss>third quarter moon</gloss>
<gloss>waning half-moon</gloss>
</sense>
<info>
<audit time="2023-04-20 18:54:15" stat="A" unap="true">
<upd_name>Brian Krznarich</upd_name>
<upd_email>...address hidden...</upd_email>
<upd_detl>harmonize with 下弦. See comments on 下弦, and eventually on 弦月.

This cannot be "waning moon".  A waning moon is the entire period between the full moon and the next new moon. 下弦 is a half moon (and even in the best case, as a bow with a string that you could maybe imagine as being a less-than-half-moon, it couldn't possibly exceed 1/2). 

This is a very old "bad" gloss, and has been the source of a lot of (I would argue) bad example sentences in the wild.

https://ejje.weblio.jp/content/waning+moon
This seems closer to "waning moon" 欠けた月

Image search will give you results with all kinds of percentages of moon visible, the restriction being that it is getting darker.  

Search for 下弦の月 and you'll get half-moons.</upd_detl>
<upd_refs>https://www.dictionary.com/browse/waning-moon
the moon at any time after full moon and before new moon (so called because its illuminated area is decreasing).</upd_refs>
<upd_diff>@@ -12 +12,3 @@
-&lt;gloss&gt;waning moon&lt;/gloss&gt;
+&lt;gloss&gt;last quarter moon&lt;/gloss&gt;
+&lt;gloss&gt;third quarter moon&lt;/gloss&gt;
+&lt;gloss&gt;waning half moon&lt;/gloss&gt;</upd_diff>
</audit>
<audit time="2023-04-21 02:03:46" stat="A">
<upd_uid>jwb</upd_uid>
<upd_name>Jim Breen</upd_name>
<upd_email>...address hidden...</upd_email>
<upd_diff>@@ -14 +14 @@
-&lt;gloss&gt;waning half moon&lt;/gloss&gt;
+&lt;gloss&gt;waning half-moon&lt;/gloss&gt;</upd_diff>
</audit>
<audit time="2023-04-21 02:04:10" stat="A">
<upd_uid>jwb</upd_uid>
<upd_name>Jim Breen</upd_name>
<upd_email>...address hidden...</upd_email>
<upd_diff>@@ -10,0 +11 @@
+&lt;pos&gt;&amp;exp;&lt;/pos&gt;</upd_diff>
</audit>
<audit time="2023-04-21 19:44:29" stat="A">
<upd_uid>robin1354</upd_uid>
<upd_name>Robin Scott</upd_name>
<upd_email>...address hidden...</upd_email>
<upd_diff>@@ -12,0 +13 @@
+&lt;xref type="see" seq="2147030"&gt;上弦の月&lt;/xref&gt;</upd_diff>
</audit>
</info>
</entry>