According to Islamic tradition:
"The deceased should be placed on his right side, facing towards the qiblah, with his head and feet to the right and the left of the qiblah. This practice has been maintained by Muslims from the time of the prophet until this day."1
This means that in Delhi, a Muslim would prefer to be buried with the feet positioned south of the head so that when his body is rolled on its right side, he or she would be facing Mecca. Humayun's cenotaph is positioned in such a way that if Humayun were laying on his right side in the cenotaph, he would be facing Mecca. In this case, the only logical entrance to the tomb would be from the south. An emperor would not want to be approached from behind, the east. Only Allah approaches the emperor from above, the north, and only the divine light from Mecca enters from the west. Humayun's Tomb's footprint is oriented in the way it is for this reason.
The sun sets in the west, which is also the direction towards Mecca from Delhi. This direction is often denoted by a mihrab. Humayun's Tomb has mihrab symbols incorporated into the jali patterns on the western facade. As the sun sets, light passes through a mihrab jali screen from the west onto Humayun's cenotaph. The sunlight splinters into geometric shaped beams of light in the form of a mihrab and crosses Humayun's cenotaph, pulling towards Mecca while the sun is setting. This only happens as a result of Humayun's Tomb's footprint being aligned with Mecca.
Set into the western wall of the mausoleum are three screens-a marble one in the main chamber of the tomb flanked by two sandstone ones in the adjoining corner rooms - that let light into the building. Inscribed on these screens, which face toward Mecca, are mihrabs. As the light filters through the tomb, these mihrabs glow against their background, replacing the words of Surah 24 of the Qur'an that are traditionally inscribed on the mihrabs of Indian tombs. Although few other tombs in India use these screens, Badauni noted in 1584 that after burying one of Akbar's close disciples, Sultan Khvaja, in a mausoleum, "which was of a newfangled kind, they put a grating facing the light of the sun, so that its rays, which cleanse from sin, might every morning falls on his face.2
1. Sameh Strauch comp., Sameh Strauch, trans., Fiqh Course: Tahaarah, Salaah, and Janaa'iz (Riyadh: International Islamic Publishing House, 2003.), 207.
2. Glenn Lowry, "Humayun's Tomb: Form, Function, and Meaning in Early Mughal Architecture," Muqarnas 4, (1987): 142.
Parodi, Laura E. "The Posthumous Portrait of 'Hadrat Jannat Ashiyani:' Dynastic, Saintly, and Literary Imagery in the Tomb of Humayun." Islamic Art 6, (2009): 129-58.
Strauch, Sameh, comp., Sameh Strauch, trans. Fiqh Course: Tahaarah, Salaah, and Janaa'iz. Riyadh: International Islamic Publishing House, 2003.