The term haṭha-yoga as “an effort or force” has very ancient roots associated not only with the ferocious cults of Yoginis, but also with earlier sources originating in the Vedas. And the understanding of haṭha-yoga, as the yoga of the Sun and Moon, borrowed by such nāthas as Gorakṣanāth and others from the kaula tantra teachers like Matsyendranāth and Ādināth. If you look at popular dictionaries, then ह ‘ha’ is identical to Viṣṇu and Śiva, especially Bhairava, which indicates annihilation. In jyotiṣa (the Indian astrology), in addition to Mars and Saturn, the Sun can also be considered as one of the aggressive planets. In haṭha-yoga, the Sun is a fire eating the nectar of life and leading to death consequently. This is all symbolism indirectly indicating the Sun. If you look at ठ ‘ṭha‘, one of the meanings is “disk of the moon” in the Monier Williams’s dictionary. Also, the bīja ठं ‘ṭhaṃ‘ in tantrism is used as the mantra of nectar, which essentially indicates the nature of the Moon. The Moon has a creative nature, and the Sun is destructive one, together they harmonise each other.