Image imported from Derpibooru.
Original: https://derpibooru.org/images/3353320
Metadata at the time of import:

Original description below this line

score_9, score_8_up, score_7_up, score_6_up, score_5_up, score_4_up, rating_safe, source_pony,
canterlot, solo, Twilight Sparkle, magic aura, choker, midnight, night sky, serious, looking at viewer, (open mouth:0.8), full body, goth, laced black dress, laced black corset, cross-laced boots, heavy makeup, holding a book, balcony, detailed hair, skinny, crown,
<lora:Concept Art 2 Style LoRA_Pony XL v6:0.5> , <lora:Anime Cold Night Style SDXL_LoRA_Pony Diffusion V6 XL:0.9>
Negative prompt: chibi, angry, moon
Steps: 10, Sampler: Euler a, CFG scale: 8, Seed: 1396194302, Size: 1216x832, Model hash: 67ab2fd8ec, Model: ponyDiffusionV6XL_v6StartWithThisOne, Template: “score_9, score_8_up, score_7_up, score_6_up, score_5_up, score_4_up, rating_safe, source_pony,\ncanterlot, solo, Twilight Sparkle, magic aura, choker, midnight, night sky, serious, looking at viewer, (open mouth:0.8), full body, goth, laced black dress, laced black corset, cross-laced boots, heavy makeup, holding a book, balcony, detailed hair, skinny, crown, \n<lora:Concept Art 2 Style LoRA_Pony XL v6:0.5> , <lora:Anime Cold Night Style SDXL_LoRA_Pony Diffusion V6 XL:0.9> “, Negative Template: “chibi, angry, moon”, Lora hashes: “Concept Art 2 Style LoRA_Pony XL v6: 09e057c225a3, Anime Cold Night Style SDXL_LoRA_Pony Diffusion V6 XL: c5be68b8f4c8”, Version: f0.0.17v1.8.0rc-latest-276-g29be1da7

As the $27^{\mathrm{th}}$ Astronomer Royal of

$$\mathfrak{Her};\mathfrak{Most};\mathfrak{August};\mathfrak{Highness};\mathfrak{Princess};\mathfrak{Celestia};\mathfrak{of};\mathfrak{Equestria}$$

I must point out that phases are literally cyclic, like phases of the moon. By a “passing phase” I presume you were actually referring to a “secular variation”, that is, a long-term, non-periodic variation, like how the moon is gradually going away from earth, and not coming back.

Also, another common misconception is that phases much necessarily be sinusoidal. In fact, any periodic variation has a phase (so it is only defined up to diffeomorphism on $\mathbb{RP}^1 \simeq \mathbb{S}^1$). For example, the brightness cycle of Betelgeuse is irregular, but still periodic, so we still refer to it by a phase.