While in reality the Earth goes in an orbit around the Sun, it seems from the Earth that the Sun moves over the ecliptic (red) on the celestial dome. When the Sun seems to pass through the vernal equinox (longitude 0°), the longitude of the Earth itself is 180° longitude.
<a href="//commons.wikimedia.org/wiki/User:Tau%CA%BBolunga" title="User:Tauʻolunga">Tauʻolunga</a>
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<span class="int-own-work" lang="en">Own work</span>